According to :
He made sure that the panel bus was cleaned, and that the new breaker
had no debris, but by the end of the day, the lights in the room went
out again.

How did he clean it?

It seems to me that if you were to clean it properly (eg: a light
touch with a metal file or good scrub with garnet paper), and the
breaker was new (such that the connectors on the back weren't already
overheated), having it start misbehaving within a day means that
something _else_ is wrong (in addition or simultaneously), or the
bus is very badly damaged (but it didn't sound like it).

Eg: inherently overloaded, or some sort of subtle not-quite-trip
intermittent fault downstream.

Swapping this circuit onto another breaker would help diagnose
the problem.

It seems to me that the loading on this circuit may be a bit high
because of the GDO. Had you operated the GDO between the new
breaker going in and the problem recurring? Inspecting all
the connections would probably be a good idea.

If it turns out that that breaker position is the problem, given
that the panel is fully loaded now, replacing the panel is just
one option. Adding a small pony panel somewhere near is another,
and it'd be a lot cheaper (vastly less labor) than swapping out the main.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.