Bad connections or bad battery?

Car battery had gone bad at least in part to corroded terminals. I cleaned up the terminals and eventually got battery to 12.8. it had been dropping below 10 volts on startup but with driving/continued charging it also started showing above 10 volts on startup.

Issue is it is dropping below 10 volts on startup again eg 9.66 or 9.86 and cutting out sometimes during driving.

I’m thinking that it’s either the battery or corroded/bad connections etc which maybe causing intermittent performance/charging.

My question is that since we known it went from cutting out frequently to being able to start frequently for some days to cutting out again, would this suggest it’s a connection issue since this implies it’s intermittent or could assuming all else is well, could the faulty battery alone have the same behaviour? If so how I would imagine it doesnt really get worse then better then worse
Unless it’s a external/charging issue?