Welcome to the forum. 
While I applaud your efforts I think a fair amount of what you say is more subjective than objective.
GD is Steampunk - hm, no.
Pace of Combat: I wouldn’t say TQ’s is bad. It is a slower paced game than GD, but I’ve never had the sort of battle with ordinary monsters that you describe. While TQ’s AoE skills may not be as numerous as GD’s they can still deal with mobs easily in Normal; Plague and Squall for example wipe the floor with mobs.
Pace of Character Progression: The main difference here is that in TQ you were/are never expected to reach the max level on a straight playthrough of the game. Therefore getting to max 75 (TQ/IT) or 85 (AE) does take a LOT of grinding. Personally I never work out builds to max levels in the game; I usually aim for somewhere between 65 and 70 most of the time. GD on the other hand is designed to reach max level because the devs want your build to be finished and not continually gaining more points as you would if the game had Paragon levels for example.
Music and Ambience: Tbh for me the only memorable music in TQ is Rock of Mages in the Secret Passage. I couldn’t tell you what the rest of it sounds like compared to other music in the game and that’s with probably 12-13k hours in the game. GD’s most memorable one, or the one that most people mention, is Lonely Moon; a very early track which surprisingly didn’t make it into the game until the Ashes expansion and the sewer complex there.
Voiceovers: Comparing what a studio with major funding can achieve with a start up indie with restricted budget. That budget was needed for more important things than voiceovers.
Presentation: Technically GD doesn’t have individual acts so yes, you won’t see a noticeable difference when moving from area to area. What we call act areas mainly come from the Grim Misadventures and also from playtesting.
GD by Acts according to the Grim Misadventures
Act 1 up to Warden Krieg
Act 2 up to Cronley or Deadman’s Gulch since iirc we got stopped at that point - not sure
Act 3 Pt 1 up to Homestead
Act 3 Pt 2 area around Homestead
Act 3 Pt 3 up to Darkvale Gate
Act 4 Asterkarn onwards
Then later for the expansions during playtesting it was
Act 5 Gloomwald up to Altar of Rattosh - again initially we could only test that far
Act 6 to end of Ashes of Malmouth content
Act 7 Forgotten Gods content
I’ll finish off by saying that the forum didn’t lose a treasure trove of info when we moved platforms. Crate migrated nearly everything except a few threads that were obsolete. There was some loss due to hackers before the migration.
Are you thinking of the closure of the old titanquest.net forum? A lot of info was lost from that sadly though the player base did our best to gather as much as possible and then migrate it to the new forum. But that was nothing to do with either Crate Entertainment or THQ Nordic as that forum was owned by another company: iEntry.