Likelihood of finding a solar installer that will allow me to DIY my own batteries and/or AC coupling?

I’m not considering batteries at first. Mostly due to cost. I want a solar install that can provide solar backup when there is a grid failure. Given that grid failures aren’t conveniently during the day and solar conditions may not allow for keeping everything I want powered during a grid failure, I’m not even sure I can have this built without a critical loads sub panel.

DIYing my own battery seems like the least likely to fly with regards to manufacturer or installer warranties. Which led me to looking at AC coupling as a potential option since it’s agnostic of the installed solar controllers, right?

Is anyone here using AC coupling to add battery capacity to their system? Pros and cons?