Where are all the high quality free opensource themes?

I originally set up my site on WordPress 10 years ago, and the theming landscape in particular has changed a lot since then.

What I am looking for, is a decent them / or theming framework, that is opensource and free to use. The other requirement would be at least to know that the project is of a reasonable size that it is likely to be updated in the future, and not become abandonware.

On the face of it, many purport to be free, but then when you look into it in more detail, they are all a lite version, that is eager to upsell you to the paid for “professional” version. My worry with this, is that it is too tempting for the developers to hold back new features from the lite one, in order to get people onto the paid for one.

The whole plugin ecosystem does not seem to have this problem – but at present, I am really struggling to find a decent theme that I can play about with / tweak / develop sub-themes from, other than those included in the base WordPress install.

Any thoughts suggestions would be welcomed.