I have quite a few recommendations. They are not quite non-freemium since they are free, but if your reason to prefer non-freemium is to avoid ads and in app purchases, then none of these games have them. They are all also open source and the link I provided below will be for their respective F-Droid version (note: some of these games are not available on Google Play Store):
Simon Tatham's Puzzle - A collection of many puzzle games ported to Android. The puzzles include Minesweeper, Net, Sudoku, and more.
This particular game requires far more attention and strategy than the ones listed above, but it can be played once every few minutes if you constantly save and reload (just don't play online):
Unciv - 4X civilization-building game, similar to Sid Meier's Civilization series.
Sherlock Zen is a logic puzzle game that looks like dos shareware from the win 3.1 era.
The previous version was one of the first android games I ever installed, I played through the free levels a couple of times but never bought the unlock.
I thought about it recently when doing a grid logic puzzle in a magazine I bought at the airport and went looking for it. I was sad that it didn't seem to exist anymore and eventually went down a rabbit hole and found the creators website which gave me huge nostalgia. He is also active on mastodon. So I bought the new version finally and have been thoroughly enjoying it!
I play sand:box, it's free without ads, but if you want some extra materials to play with you have to pay a dollar for premium, i never get bored of it.