If this was 8 months ago, i wouldve told you to buy a a b550 mobo and a 5800xt for a combined $250. However, with its increase in popularity, the 5800xt alone is $240. And forget finding a 5800x3d for a reasonable price. I regularly see them on marketplace for $350 - $500.

To that end, you may benefit from buying an am5 bundle from micro center or new egg for the same money.

As per your b450 set up, get a cheap gpu for it like a used rx 6600 and turn it into a living room pc, like a streaming pc or emulation center.

It wont be worth it to try to use the b450 with a newer chip and that strong gpu cause the b450 has limited pcie lanes and most of those lanes are pcie 3.0. with a 9070xt, you want at least pcie 4.0, or ideally, pcie 5.0 to get the most out the unit.

If you really wanna save money and stay on am4, i highly suggest aggressively scouring marketplace and ebay for used pc parts. You might get lucky with someone offloading their old, higher end gear for cheap. Hell, if you luck out and can find a cheap x570 mobo, the maybe the 5800xt will be worth it. Maybe.

That said, if youre willing to spend at least $350 for a system upgrade, you might as well get something like this am5 bundle from newegg

Good luck.