Holographics, psionics, etc. do little to nothing against reaction fire from two dozen aliens
Actually, they do. Your psi-actions are not reactable. You can mind control a puppet or two, and bait some fire with it. Among said dozen probably a few are armed with pistols, and are effectively harmless, so you focus on those who are. Puppets, summons and drones are freely expendable reaction baits.
Here's my playthrough of the Arbiter UFO. I guess it's one of the harder missions in the game.
https://youtu.be/BsB0o_PfP0o?t=1138I start by directly staring onto sectopods. By turn 4 or so most of my troops are outside the Ironfist, and are storming the UFO. I'm limited in movement by turrets here, but as soon as they're done, I'm out.
In nearly all missions, as long as there's no constant enemy flow, there's no problem dispersing. With psi-scouts and motions scanners you get pretty good map awareness as to where to expect the enemy.
You're not even supposed to have the Avenger until the last couple of missions or so. With door-less Ironfist you simply can't camp.
Blaster bombs can hit anywhere on the map and Infernals and Spartans have the accuracy to hit anything from spawn.
I've been blastered exactly one time this campaign, losing one guy in the process.
to consistently avoid substantial casualties on every mission without scumming
Excuse me, but this sounds like words of someone who never tried, or even weighted the risks. How many casualties would you deem acceptable? Mid-to-late game is where I usually have the least amount of casualties because of all the stuff you get. I lose maybe a dog here and there or a couple of guys.
But even then, in late versions 1) spartans make too good of soldiers right from the get-go 2) you can just stamp vets via coupons. So even in the impossible event that you wipe a few times, you'll always have a decent team to bounce back to. And even if we ignore that, the casualties have never been a campaign-ending matter.
If you want to keep safe some specific soldiers for roleplay, there's also ways to do that - I manage to keep alive a few "original members" each campaign to give them specifically the task of ending the alien brain.
Opening the Avenger doors to a red 10 for the 6th time in a row just makes you reflect I guess, just wish there was a lever besides the "add more aliens" for map makers.
Well, why is it "6th in the row"? AFAIK, there's only one "big" UFO that you're required to do (Arbiter), and the rest are optional.
I agree that more enemies don't make the game more interesting (cough cough three hours MIB base anyone? You can (and should) smash and grab it, though).
But be specific. What do you have in mind for which endgame encounters?