> only 20 community supported devices

You realize it's a result of reverse-engineering done by volunteers? To me it looks like a large number actually. The main hurdle for true "Linux Phone" is the lack of free drivers and specs, such that volunteers must spend tremendous efforts to make basic things work.

> which Linux phone should I buy and how could I contribute to its success?

To help Linux ecosystem on mobile, you could support vendors who intentionally provide free drivers or specs and help improve the software running on their devices. AFAIK only Pinephone and Librem 5 fit here. The former is in stock from time to time.

Also many users of GNU/Linux phones run Phosh developed by Purism. Helping to improve it would help the whole community.