I found all of the IKEA buttons quite good and they are really affordable. They are not FOSS like you requested but they use ZigBee and they work really well with HA. The light bulbs are good, too.

I still use a ConBee II for the ZigBee connection but I think there are better options available now. I bought it years ago.

I’ll just say it: PHP. I mean the language itself is not really pretty (but came a long way).

But I’ve got a nice side gig running where I help small businesses and schools to keep their old PHP based websites running and secure until they can afford a new system. In that area money is often very tight and there are no “content managers” but a few people maintaining content with next to no technological knowledge and in their spare time. So changing systems is a hard task. Being the really decently priced guy who keeps their PHP ship afloat is pretty rewarding (not really monetarily but personally).

So working with PHP I have a very nice group of customers who are grateful for every bit of help. I love that.