It’s called Kii. You can still get a version from a 3rd party site since it hasn’t been on the play store for years.
I’m not sure how much I trust those apk sites, (so downloader beware!) but you can at least see the screenshots: https://www.apk4fun.com/apps/com.zl.inputmethod.latin/
I don’t think it’s anything earth shattering, but it’s kinda nice, and fairly minimal, and it’s what I’m used to, so I’m happy with it.
Have you tried other options like AnySoftKeyboard or HeliBoard (fork of OpenBoard)
I’m pretty sure you’ll like AnySoftKeyboard, it’s pretty minimal and similar to your current keyboard. And it works with Thunder.
You can still submit a bug report for Thunder not working with Kii, I’m pretty sure the Thunder devs would be willing to fix this.