Have you seen VyOS? It’s a Debian based router OS that effectively acts as a wrapper for FRRouting. It’s very good.
I like Cisco hardware. And if it was purely a question of routing packets, I wouldn’t consider an alternative to IOS. But… if I could run a *nix of some sort on Cisco hardware, that would be pretty neat. Opens up some other possibilities. (I REALLY wish I could just get something like a raspberry pi in a WIC card format. Cisco does make something similar, but the software is horribly locked down. I’ve rooted one, but… that didn’t really gain me anything. Everything about those Cisco boards is uncooperative.)