I'm happy with my Pocophone F1. It's not the only phone with very good ROM/Free software support, but among the cheapest (around 300€, maybe a bit less by now, it should now also be available used).

It has lots of RAM, an SD card slot, a headphone jack, a high-end processor, a large battery and most importantly is very repairable (a screen costs about 20€).

The initial unlocking process is a bit annoying and stupid, but after that it's quite nice. The only thing you could say it really lacks is NFC.