(As long as we aren't talking hard sci-fi) the Science in a book isn't about science itself, but a plot device. The super-hyper faster-than-light drive of your space-ship isn't anything different from the horse cart in your fantasy world. The difference between the world-ending black matter torpedo and the graviton hyper ray is the same as the difference between a fire ball and water magic.
As long as you don't write for a very specific subset of readers, most won't care about your science at all. Even more so, a lot of readers won't even bring the knowledge needed to understand your science in the first place.
Important are the same two things as with every kind of magic and worldbuilding:
(1) what are the rules
(2) what are the implications for the story
If you introduce faster than light travel after 200 chapters, the reader will want to know why it wasn't available before. If your new weapon can destroy planets with ease, why was the last base of the enemy "the worst threat to the galaxy ever"? If you can jump from planet to planet, why not directly jump to the final goal but travel through all those worlds in between?
Those are the questions you should answer for your reader: what are the rules for using the technology and how does it fit into the story. And yeah, you can go a bit ham with some techno-bubble for your the rules, but the (majority of) reader(s) won't care whether your "once a month weapon" is due to "mana-limitations" or "graviton-van der waals-exchanges in a sub-vacuum". The important fact is "once a month".
So make the cost clear, make the rules clear, and explain the impact to the story/world and how it fits in. But don't try to teach science to an audience that would need years of math classes to be able to understand the theory behind it.
And yeah, there are people who really like hard SciFi and it can be really well-written and entertaining. But I doubt wether you find a lot of those readers when you combine it with a typical isekai story. So given your setting, I would go for as less actual science infodump as possible...