it would be nice to have OpenWrt on this fritzbox 5530
Somewhere between unlikely (at least considering its main features) and a long road with an unclear ending ahead.
Woohoo! PRX300 based. That's the MIPS based 3rd generation SoC from Intel's former Home Gateway Platform Division which they sold off to MaxLinear a few years back. Haven't actually seen many real devices with those.
There is actually a whole thread about the abandonment of that MIPS InterAptiv stuff...