Hey, welcome back to Gears! I played a lot of Gears 4 Horde too, and was a big Scout fan myself. So I feel like I can reply to some of your questions/suggestions.
Anyway:
1: I don’t think that the classes will ever be consolidated into fewer classes, because of the existence of Ultimate Abilities. However, I am fine with that, because the Ults are each unique and help provide a lot more variety than Gears 4 ever did. And sure, the classes are a bit more specialized and not as well rounded, but there are at least a few different viable builds and ways to play almost every existing class (promotional classes need extra work, though).
2: As a Scout main in Gears 4, I can confidently tell you that there are at least 2 particular classes that can play a bit similar to the Scout class from 4. These are:
(A) Inflitrator.
-This is the Shotgun class for Gears 5, with much more powerful damage output than in 4. It also has similar cards to increase power pickup radius, and also to heal from power pickups, just like the Scout from 4. It doesn’t get extra power during the round like Scout did (good, because that was too meta), although Inflitrator USED to have a passive to get bonus power from close range kills (a better way to balance getting more power and incentivize playing the class correctly, IMO). Lastly it expands upon the limited cloak invisibility cards from 4, and with a stim build can get nearly as tanky as the Scout was while being able to easily replenish the stim with kills for extreme sustainability.
(B) Brawler.
-This class is the CQC tank class in Gears 5, with a bit more focus on special melee abilities and optional fire builds as something new. Brawler is notorious for being the most inherently tanky class in Gears 5, with cards for increased health, faster healing, and damage resistance to melee and extra resistance while sprinting, similar to Scout in Gears 4. This time, in Gears 5 as Brawler, the class has unique melee abilities to help it stand out; tackle enemies after sprinting for a short duration (kinda like shoulder charge or bashes from other games), and a sucker-punch basic melee that does extra damage and briefly stuns or staggers enemies. In addition to all that tankiness, tackle kills INSTANTLY heal and give stim ON TOP of all that, allowing for extreme survivability out on the field.
3: There is at least one map each for Horde and Escape that has manned turrets, although yes, they are under utilized. More fortification options would always be awesome if they could ever add more.
4: I agree, I really miss the 25 wave Horde in Gears 4, this was the original version of Frenzy, and was the best version of Horde IMO. Not too long, not too frantic and short, with longer, condensed waves, and a boss every five waves. I so hope that they bring it back to Gears 5 eventually.
5: Most classes already have a large variety of skills, although new unique ones should never hurt. However, a few classes, specifically the promotional classes, really do absolutely need more cards and buffs bring them up to par with the rest of the classes.
6: I think that most of the passives are pretty great right now, although I do agree that the engineers need something a bit better, especially for use in Escape where there are no fortifications. Also, once again, the promotional classes really need some new stuff.
I hope that this helps you out, glad to reply!