To get a "live view" (you're right, astro cameras suck compared to DSLRs in this regard), I use 2 second exposures at gain 500 and loop them.
That's a rough focus. Then I drop the gain down and start dialing in fine focus with the Bahtinov mask.
3 mins should get you stars at least, even with the L-Extreme. But in terms of nebulosity (or whatever you're taking pictures), remember that some objects are FAINT and will require more exposure time. What were you trying to shoot?
And you might need your program to auto-stretch your subs to see the target. Not sure if APT does that, but NINA (amazing!) does.
If it helps, I was shooting Carina with 4 minute subs and the L-Extreme filter in Bortle 5/6 skies.
Also, as far as I know, "unity gain" doesn't exist for this camera.
The 2600MC is a fine camera, let's try and get your problems solved
