16 years ago, a man named Eric Sink decided to document the process of creating, marketing and selling a small piece of software he had always wanted to build, "Winnable Solitaire" which is, as you describe, always solvable.

His series of articles coined the term "Micro-ISV" which was a once-common term for a small software company with approximately one employee. A category that is now covered under "Indie" developers.

Here's the article that started it:

https://ericsink.com/entries/Announcing_Winnable_Solit.html

(He didn't end up selling many copies, but we all learned a lot on the journey, it was very interesting at the time.)

The best collection I know is https://www.chiark.greenend.org.uk/~sgtatham/puzzles/ which has ports for various platforms (in my case using android, but apparently there's an iOS port too, see links at the end).

The games have various difficulty levels to choose from.

I’ve recently learned about Spider Solitaire, after getting Clubhouse Games for Switch.

That game seems highly unsolvable most of the time, and have thought many times how it’s be great if there were a version that generated only solvable seeds, as you can sink a lot of time into that game for it to only be unsolvable.

For example, I finally beat it after probably over 150 plays.

Then again, it’s also easy to get yourself in a jam from your own decisions, and I probably am just bad at it.

I think Flipflop Solitaire is supposed to be always solvable. It's also very well made.