Sure, x16 Gen3 slot is fine with Gen4 GPU.Maybe you just get a x16 GPU from AMD than upgrading your motherboard (going from b450 to b550 for example makes no sense), i dont think 100 frames difference in CS GO for example is nothing and there are a few games there that lose a decent amount of FPS (who knows about the future games) and xx60 cards are all about 1080p.
For B450 to B550 transition, that depends. There are high-end B550 boards, such as Pro Art from Asus, with more premium connectivity, the first ever B550 board with Intel's dual Thunderbolt 4 ports and two LAN ports.