Tsunderes simply CANNOT exist in a setting where the guy is smart, capable, and mature. Said guys instantly transform Tsunderes into flirty school girls honestly crushing on them, instantly eliminating all Tsundere-like aspects of them. Any girl who manages to keep those Tsundere behaviors with a smart, capable, and mature man is just a psychopath. The behavior of the male side is every single bit as important as the female side in order to make the Tsundere mechanic work.
It's really silly and incredibly biased to say that they can't exist in a setting where the guy is capable and all that without being a psychopath. Just because he might be the exact right kind of guy for her doesn't mean that she will be comfortable with her own feelings. Maybe she's had negative relationships in the past and is worried that things are too good to be true, so she puts up a tsun act to try and create distance in an attempt to protect herself (yet likely hurting herself more than protecting herself). Maybe she's in an important position, such as a noble or the heir of some important business or something, and everybody expects her to get married and settle down, yet she wants to stay independent and is afraid that she's just giving in to everybody's wishes if she goes down without a fight. Or maybe she's just really fucking awkward and has never had romantic feelings before and doesn't know how to handle them. Or maybe she was raised in a negative environment and she's just treating her love interest how her parents treated each other. All of these are at least somewhat realistic things where, as soon you give them an anime art style, pass as "tsundere" in the modern context of the word. Or, if having actual motivation (read: being a good character in general) is all that it takes to make somebody a classic tsundere, then they would pass as a tsundere in the original context, too. And just because a girl has flaws that make her uncomfortable in a relationship doesn't mean that she's a psychopath. It means that she's a real character who has flaws. Obviously, the tsunderes who are hyper-violent
do have some major problems (and are closer to yandere than tsundere most of the time), but I almost never even see those anymore outside of gag anime and I watch a fucking ton of anime.
Now, while I will agree that the tsundere should eventually get rid of these tsun-ish traits (or else she's stuck never being truly happy with herself and her relationship), it's a pretty massive stretch to say that it instantly happens or to imply that it should instantly happen. The way you phrased it just makes me think of a reverse "bad boy" trope where Mr. Lucky comes along and somehow instantly transforms the tsundere at first sight into being "good" for him. That basically just removes agency from her and implies that she's not her own person with her own personality and reasons for her behavior - that the only reason she's tsundere is because she hasn't met the right guy yet. The rest of your post was good, in regards to a very specific type of tsundere with a very specific motivation for being a tsundere rather than speaking of tsunderes in general, but I have major problems with this paragraph.
Personally, when it came to writing my favorite tsundere character, she was defensive and jaded because of being raised by an overprotective father which led her into entering relationships with people who she really shouldn't have gotten into relationships with to rebel against her dad. Naturally, these relationships were bad experiences that only led to her being distrusting and disliking of men even more. Even when she finally did find a guy (and a girl which made her more comfortable/secure) that she liked, she still struggled with her feelings, tried to put distance between her and him (but was not a tsundere toward the girl), and then eventually caved in and admitted that she didn't like how she was before promising to try her best to be more positive and less verbally abusive (and at this point, she already improved to the point where she was basically a normal adult but was still self-conscious and overly critical of herself). Now, she's not a tsundere at all, but she'll still give a bit of stereotypical tsundere fanservice because she's comfortable doing it in an ironic sort of way and because her partner likes it.
And just for two cases of excellent series where the main character is considered handsome, competent, intelligent, and capable: Trinity Seven (albeit the main tsundere is typically played in such a way for laughs) and, far more importantly, Chivalry of a Failed Knight. While I can't speak too much in regards to Trinity Seven since the tsundere there didn't progress that much, Chivalry's tsundere and the MC had fantastic relationship progress, character growth, maturity, and a gradual and believable conversion from tsundere -> deredere. She was definitely tsundere and the change definitely wasn't instant just because the MC is a good character for once rather than some hyper-dense harem protag.
tl;dr: it's silly to say that a tsundere can't be a tsundere if the guy is a good character instead of a bumbling idiot. Guys can't just magically make all of a girl's insecurities / problematic tendencies disappear just by being Mr. Perfect. This is like saying "bad boys" are only bad boys because they haven't found a caring, kind, intelligent, mature, and attractive girl yet. That's alright for pure wish-fulfillment where you aren't looking for very complex characters and are instead just trying to satisfy a fantasy, but I would hope that no character instantly changes who they are because of some guy/girl in anything else.
And now to answer the OP:
Anybody who writes tsunderes, how was your experience doing it?
It went great. Here's how to do it.
1. Make them at least partially realistic by giving them actual depth and reason to why they are the way that they are. This is much easier to do if you've ever known anybody in real life who was overly defensive because of past circumstances or inexperience.
2. Make them a well-rounded character who isn't tsundere 24/7. Show off their other sides, especially the ones that are easier to sympathize with.
3. Treat the tsun-ness as a character flaw to be overcome through development and effort on the tsundere's behalf. Another character, such as a love interest, can be the catalyst that sparks their desire to change, but the change still has to come from the tsundere themself. It is impossible for somebody to truly change as a person without wanting to change on their own. You can't change somebody else. You can help somebody change, yes, but that's only if they
want to change in the first place, and they still have to do 95% of the work to truly change.
4. Don't make them physically nor mentally abusive. At all. Just don't unless you're planning on writing something that deals very heavily with the psychology of such relationships while treating it in a very serious manner rather than as an anime trope. At that point, you're less writing a tsundere and more writing a yandere. Verbal "abuse" can be fine (such as calling the love interest an idiot a lot, for example), but it shouldn't be too heavy, should still be treated as a problem, and shouldn't be normalized as some perfectly healthy and normal thing to be doing.
EDIT: Or, alternatively to the above steps, just make her a normal girl who likes to act like a tsundere while actually not being one, and their love interest is in on it. They're just a perfectly normal person who just so happens to enjoy roleplaying as a tsundere, effectively making them one for the story since it's what the audience sees the vast majority of the time, while their love interest is in on it and preferably enjoys the act. Then they can be as verbally abusive or physically rough as they want and all that because they have their partner's consent and enjoyment. Consent is a very powerful thing.