There are multiple tiers of batteries, I got the $50 one I linked but the first hit was $120 on their site.

So you can find the model’s battery here, and then google that and batteries plus. I have an 850M model which uses the rbc17 model. I’ve highlighted that on the apc site on this photo:

Batteries plus had a Duracell version of the one my APC unit needed, in stock in store so I could get it same day. If you’re in the US, might try them.

Which hardware is recommended? Trying to search through their list a ton of it is already on old-stable and seemingly ready to be eol-d. I’m not really interested in spending on new routers playing whack a mole with security updates every 2 years. I’d rather have poor performance and a longer lifecycle versus unknown likely marginal support window.

I use pf as the firewall on my server, what is the difference/reason for the opnsense fork?

They also list my current one as unsupported unfortunately, I think because of the Broadcom WiFi chip in it.

That was one of the things I was considering was whether running a router before the WiFi and then just using them as access points might be a better choice.