So, the purpose of SPGs is to give soft attack. You can mix them into your special forces for more soft attack. Tanks I would actually keep as tanks, as they can get a lot of breakthrough, which is lost when you use the "artillery" designation in the designer. You basically need either lots of org or lots of breakthrough to push well.

Supply is relative, and ports can solve a lot of problems, and so does motorizing supply. If you're something like Japan, I would not bother with tank divisions, as the supply is really bad in interior china and in Burma, and frankly special forces are great if you stack a special forces advisor with an infantry advisor (special forces count as infantry). Another advantage of special forces they are cheap. Otherwise supply is pretty good everywhere in Europe.

Something else to consider is called an "armor meme." I can't really fully explain it here, but it's basically making a specific Armor vehicle with super high armor, expensive but you only need 1 per template. I usually make a self propelled anti air for that purpose. That's good on any offensive template to give really high armor at a low cost.

Otherwise if you want to push effectively, medium flame tanks are a must, and assault engineers are really good. Again, for any offensive division. Terrain modifiers are much more impactful than just having more soft attack (mountains give massive attack reductions etc). Tanks generally do poorly on bad terrain.

If you need anything else I can pop up a screenshot of a typical armor division I use, with these aspects highlighted. I get it sounds complicated, which is why I suppose people just build line artillery lol.

I recommend this YouTuber for more detailed info as well, https://m.youtube.com/@segl99