Should I avoid placing them in structures then? I tend to lazy out and plunk them into whatever house is nearby...
Absolutely. Only place in a structure if it's in an immediately strategically useful spot (for example, covering your flank while you advance your main army). Don't just leave it covering an attack route that has no value to you at the moment.
If you read my tips you might have seen
this one explaining why it's so important to bring MGs (and your whole army in general) along. They're actually more vulnerable in a building alone than they are with your army.
I like your blog with all the tips and tricks, yeah I tried the Maxim Su85 combo but it seemed to falter when faced with panzer grenadiers and panzers. Needed more micro tbh, and better positioning on my part.
The build order of your opponent is also very important. I've played with the AI only a little bit (mostly to get the costs of units and abilities) but if i recall correctly, they tend to aggressively cap.
You said they get mortars, scout cars, PGs and grens.
These two builds are dramatically different:
1. pioneer, pg, pg, scout car, gren, gren, mortar
2. pioneer, gren, gren, mortar, pg, pg, scout car
Despite having the same unit composition, you'd play VERY differently.
#1 would mean you should cap aggressively. Head to cutoff points (especially for their fuel) and force early fights. I would also use a triple conscript opening against that vs. Maxims. You'll have a dominating opening before his PGs hit the field, then if you blob around you'll always have the advantage. Build fuel depot, head to T3 so you can get T34s to finish the game early.
#2 The maxim opening will be fine. Play defensive, take "your" 50% and head to T4 if you think they're going P4s.
I'm not saying that the AI is necessarily doing either one of those, but just giving you an example of how they differ.
You can use the post game build order tab to check what they built and in what order, and use that to prepare for your next game.
What doctrine would you go with? I'm guessing HTD would be useless.
Guards Motors for maxim spam. Guards rifle for conscripts (PPSh will let your conscripts actually fight PGs without getting wrecked too hard).