I think you might be exaggerating that a no upgrade/pp/parts run would have an abnormally high turn count and death rate and miss most/all skill points later in the game. I'm up to sc 28 on a no upgrade/pp and low turn count run and I haven't run into anything particularly difficult yet.. no allied units destroyed either.
I've been playing all routes and I've noticed that the real routes are both more difficult and give worse part rewards.
I am using parts, but I don't think they have a huge impact on the difficulty level overall. That might change later in the game with the more powerful parts like platinum emblem and that item that gives a free use of Love seishin once per scenario.
First half is pretty easy, aside from a few odd units like the Firebugs but they really bring it up towards the end of the game in terms of number of units and strength. It's not as brutal as it could be but it's certainly a more work than the first half. Some of the bonuses given to the bosses are pretty interesting and can surprise you if you aren't paying attention.
Stage 28? That's still first half, and I think most of us would agree the first half isn't that hard.
Now, LATER ON, you'll probably have a hard time. Do I know when the difficulty kicks up? I have no idea. I only realized the game was getting harder when Chirico was starting to get into situations where he started to encounter grunts who could one-hit kill him.
Its not really a matter of it being HAAAAAARD, but its more of a war of attrition later: high HP enemies (10k+), large waves of enemies, how most of the Skill Points require you to kill either a single wave of enemies (40-60 enemies), OR kill a boss unit (who usually has 50k to 100k HP, depending on if its a real or super unit), by Turn 3.
Hell, later on, its kill the entire wave THEN kill the boss by Turn 3, is the norm. There's one stage later where you have to kill 6 boss units in the SAME TURN, but thankfully, there's no time limit for that one.
I'm near the end (still!), and nearly every unit I have has Continuous Action, Ignore Size Penalty, Predict, and Will Limit Break (seriously, nearly all of them, even the dinky support units and motherships). Every unit is at least level 5 upgraded in all of their stats. The only units I have that are max weapons at this point are Amuro's Nu Gundam, Wing Zero, Chirico's units, Crowe's OG unit, Shin Mazinger, Shin Getter, Shin Getter Dragon, and Gurren Langann. Only Wing Zero and Crowe's mech have FUB. And those units I just named that have max weapons are the only ones I equip items on.
Even with all that crap, I'm STILL getting frustrated by these final stages. Not because its hard, but because they have high numbers, and they keep throwing waves at me, even though I should be used to the fact that in after playing over a dozen different SRW games, wave after wave should be a norm I should expect, BUT NEVER DO. My ONLY consolation is that at the very least, I have at least learned to not spend my SP like there was no tomorrow.
Yes, the above is pretty much a rant, because a few days ago, I thought I beat the last wave in the stage I was on, but LOL NO, they threw another wave at me. I put the PSP down at that point, because I needed to calm down, lest I throw my PSP against a wall.
Obviously, I haven't calmed down yet. ^O^ And I'm so close to the end (I'm on stage 58).