The big picture on Planetfall combat is that it’s all about the special abilities, mods and the synergies between them. The biggest hurdle while learning the game is not knowing how exactly the enemy is pounding you into snot: they may be getting bonuses from their mods and abilities, they may be debuffing you with mods and abilities, etc. etc.
My suggestion, which I detailed way up thread, is to pick a fairly straightforward faction like the Vanguard or the Amazons, comboed with a suitably synergistic tech (Promethean for Vanguard and oddly Synthesis for Amazon) and then learn THOSE mods, abilities and synergies. You still won’t know what the enemy is doing to crush you, but you will learn how your own units work. Then you can gradually expand out to other combos and learn as you encounter various opponents.
The tactical game in Planetfall features a truly massive learning cliff but I feel like once you have climbed that cliff, it is also one of the best tactical games.
A few tips to help the climb: getting access to mods or operations that can cleanse status effects is one way to respond to the enemy screwing you over. The Prometheans have good mods and ops for that. Also, self-healing can be a huge way to avoid that “inevitable unit loss” you complain about - the Amazons have that, and both the Prometheans and Synthesis have some options.
As for the issue of being ground down by being forced to attack the enemy, you can use cover and overwatch to force the enemy to come to you on many maps. Learning which of your troops have overwatch is key. For example, your basic Vanguard Trooper, as weak as it is, is actually quite an excellent little core unit b/c of its range and overwatch. The Amazon core Hunters are also better than they seem, but only if you keep them in cover (they gain range from being in cover.) The range of your attacks and abilities is also a big deal. The Vanguard Trooper with a base 7 range is actually quite excellent for a tier 1 unit. Etc.
Things to avoid: in most cases, cheap melee units are pretty much redshirt fodder. There are some exceptions but relying on them too much leads to attrition. Melee can be excellent but works best on faster or more survivable units.
Another starting tip: choose a Hero with the package that includes the old Imperial ATV - that thing comes with a once per battle healing drone and that healing drone can dramatically reduce your early game attrition. I believe all races and factions have that option, although it does cost 2 hero points (but it’s worth it!)