I don't think you should sell your ER6P. What might be more interesting is to build your own images from master since master is (or can be) always in flux. There's times where it breaks, and there's times where it's stable for months. Right now the iptables 
nftables migration is in full swing e.g., in master, and I believe developers are gearing up for a new release branch (push to the new 5.10 LTS kernel hints at that, amongst others).
What I tend to do is (when it's technically feasible) backport support for the device in question to the latest stable release and build an image off that. From what I can tell, the patch adding support is rather trivial all in all. So that looks like it could be easily backported to 21.02 (although the changes of it being accepted are slim). I could take a stab at it and provide you with a patch, if you'd like. Then you'll just need to build your own image off 21.02.
You're aware OpenWrt does not support the passive PoE capabilities on the ER6P?
The Octeon target (which supports the Cavium SoCs) suffers from some hard to track down memory leak with kernel 5.10. See also here.
Marvell bought Cavium so they own the IP now but Cavium hardly bothered with upstreaming support, and it looks like Marvell isn't really doing that either so far. Make of that what you'd like. I personally sold my ER4.