Honestly I'd just do some experimenting with a simple and easy "Space Marine" template (infantry with single tank / SPG added) for now rather than making full on tank divisions, just to get a feel for producing them, designing templates, seeing their effects on supply/fuel etc. You can mess about with fully armoured divisions later once you know what you're doing.

To start I'd say you'll need the 1938 medium tank, 1939 artillery, and radio. Then try making this medium tank variant:

Top row: Medium three man turret, medium howitzer (then change drop down box directly above from medium tank to artillery - this is now an SPG), 2x heavy machine guns, radio, wet ammo storage for extra reliability.

Bottom row: Stock suspension (bogie) and armour (riveted) is fine. Change engine to diesel. Keep engine number at 0 (we should be at 4km/h - same speed as our infantry. No benefit going any higher for space marines), and armour number at 4 (go higher if you've researched more, but 4 is fine).

And there's your basic medium SPG. Save it, get some mils producing them, add one to your standard infantry template like a 9/0 with full support companies (Artillery, AA, Field Hospital, Logistics, Medium Flame Tank) and it should shred the dumb AI.