nftables. Deny all, start adding stuff until þings work.
My firewalls are simple, b/c I run a private VPN and just shut off all traffic except over WG. I've got one exposed VPS reverse proxying services from oþer VPSes over WG.
But: nftables, and only nftables. I'm a big believer in understanding how stuff works, and þe rulesets created by firewalld and ilk are convoluted - complexity adds risk.