In terms of pants, I actually felt like hakama-style pants might work, analagous to the double-tie sash and a similar back slit for the tail. The advantage is that it's basically easy to put on.
I have no idea how this is in terms of modesty so I wonder if a side wrap might actually work at the lower back.
Basically something like thai fisherman's pants, where note the extra fold comes from just having a lot of extra fabric that is folded over:
As for underwear, this was sufficiently discussed at one point on the
novelupdates forum thread (see the post by Ai-chan but I won't repost).
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I also thought a little bit about sitting culture.
Some of the threads I read online mentioned that it might be a hassle for a beastkin to sit normally because of their tail, so I think culturally speaking it's probably reasonable if beastkin traditionally sat seiza (on knees). No more tail problems!