How far have you gotten on endless mode?

I am currently on my 11th sector, after facing the Unknown twice.

I chose the BSE Harbinger as my command, and two BSE Valiants as my escorts.

Between the ships I have
6 death ray and 4 mega plasma cannons. All other facilities I opted for shield boosts. I have four shield drones and four repair droids in my hangars. I haven't lost yet and feel as though I won't until I choose to end it. I have absolutely everything maxed on upgrades as well

I have made it to around map 23 (don't quite remember). Eventually you will get unknowns, and in the last sector all their bases will use laser cannons with super long range. They will vaporize one of your ships before your weapons have even loaded. I have not found a way around this yet.

This is most likely a death ray, the longest range and most deadly laser weapon. I'd advise having a similar setup to mine, with maybe laser drones in the furthest forward slot, as they will hopefully take the hit first. I will keep going until I've seen this myself.
Another strategy might be to set off a distress beacon before going in to thin out the amount of ships there

If you have already removed these strategies, I am at a loss until I get there myself and figure it out.

I think I hit about 28 before I got bored and just played sloppy until I got wiped. I had found that the power creep with found weapons meant that Death Rays and Mega Plasma Cannons weren't as powerful as you'd think compared to other weapons. My strategy was to always have some missile weapons in addition to various guns and beam weapons so that the missile weapons in combination with my bomber fleet would absorb the worst of the enemy incoming fire while my gun and beam weapons did the real damage.

I too got into the mid-20's and then started getting sloppy until I died. Like Draikan said, eventually you get a bunch of lasers/death rays that torch your fleet instantly. It's against celestials or unknowns. The only way to fight it is to distress beacon all the time and attack enemies while they're still charging up their weapons after warping in, but it's an incredibly tedious way to play... you have to do it all the time since you don't know if the 3 ships in the next area have 6 death rays or 6 ion cannons, so you have to expect the worst.

I have made it to map 31, but it is impossible to beat it. I always use 3 Valiants. The command ship is configured with every slot but one filled with fully buffed shield boosters. The last slot has my most powerful shield drainer. The hangars house buffed repair drones. The other two ships have every green slot filled with buffed shield boosters, and the red slots filled with my most powerful beam weapons. The hangars house buffed bombers. But every time I enter the last sector, I lose one ship nearly immediately and the other two usually before the engines recharge. Once I was able to save the command ship, then I had enough scrap and resources to buy and equip two more Valiants. I went back, hoping that the nearest one had been weakened enough to destroy, but I got killed anyway. The distress signal gambit won't work because the last sector has four bases in it. They don't move. These four bases are armed with super powerful, super long distance beam weapons. I have a trick for saving my game at any time and restoring to that state.

I am now currently on map 73. I still use 3 Valiants, but all weapons now are beam weapons. 3 Tesla coils and the rest all laser cannons.

I'm now on map 82. Map 80 was very strange. No hostile ships except for 5 capital ships that warped in to attack a battlestation as part of a mission to protect a battlestation under siege, and five or six enemy stations.

I had 40th stage on Easy Endless. Im now on 27th on normal endless