smpl

Your system is fully updated from at least the kernel/initramfs and up. Next you’re running a system that has additional security measures.

So this breaks down to: What is firmware and are you aware of any issues in it? If no then there’s no reason to get a new phone.

I’m not aware of any firmware security issues for any Android phone assuming firmware is pbl, sbl, aboot, modem or on-chip and even if there was they would be hard to exploit given your up to date and hardened system, but that’s all theoretical and also apply to any new phone you would purchase.

Alternatives for flashable smartphones are Fairphone, Nothing Phone, Teracube or any Murena.