Is it just time to turn it off for the winter?
Is there a way to cut connection in the dark so the controller doesn't flatten the battery?
Are/can home built charge controllers be more efficient?
Would adding more panels give me any spare capacity at this time of year?
Even if you do turn it off for winter, if it's outside and a lead-acid, I would charge it fully first. Most solar systems are protected enough for you to just stick a normal 2A car battery charger across the battery without disconnecting it. That's what I do, when the voltage drops below about 11.8V I put the car battery charger on it overnight. Repeat as needed.
The EPEver has a handful of low-cut off and low-restore voltage triggers, however I wouldn't trust them. Mine is set to cut off at 10V, but last year the battery collapsed and the charge controller didn't connect it until it hit 8V.
The issue with making disconnect circuits is they have to take the full DC amps available at the battery fuse which leaves you with a large power mosfet or a relay. You have to account for the voltage dropping under load and relaxing back up when you disconnect it. You can also risk the charge controller remaining off due to disconnect but it's a rare bluesky winter day.
Home MPPT charge controllers are feasible. It's just a buck converter and scanning algorythm. Jullian Ilett on Youtube did a series where he built one. The EPEvers were cheap and featured enough that I didn't bother. Though I was going to revisit it just for fun.
Adding more panels would give you more capacity, but also more loss in summer when you can't quite use it all.
You'd be better off making sure the panel is mounted high as you can and angled up nearly straight for the winter sun. Battery capacity is more practical.
That said it really comes down to the amount of energy you can get in a solar day, what the weather is like and how long your system will run on it's battery with the limited top ups from the panel in winter.