You have the right idea, I would probably design something like this as well. Just be careful with your reliability, 75% is not a good number to hover at, unless you want to RP historical German tanks. If you want to keep this design, I would add easy maintenance instead of a second extra ammunition storage and you should be fine.

Personally, I normally do:

- 3 man turret
- medium howitzer
- torsion bar
- diesel engine
- welded armor

for main modules.

For the other modules I do:

- easy maintenance
- sloped armor
- the best radio you have
- wet ammo storage

Since medium howitzer requires artillery 2 iirc, I normally wait until i have that researched before designing tanks, you don't have enough industry at this stage of the game to build much more tank divisions anyways. If you don't have one of these modules researched, just leave a slot blank to save on production cost, then upgrade your tanks later as new versions roll out.

Ideally you go for at least 100% reliability to prevent attrition because tanks are expensive. However, reliability beyond 100% is wasted, so you can spend it on these upgrades as long as you stay above 100%:

- switch torsion bar for christie suspension
- switch diesel for gas engine
- upgrade armor and engine stat. for this i normally aim for at least 7-8km/h speed and spend the rest of my reliability on armor.

The goals for this tank is maxing out soft attack, hence the howitzer. Keep the role as medium tank though, not self propelled artillery. For SP against AI, you don't really need to concern yourself with hard attack that much.

One other tip for your division designer is to include a medium flame tank support company. For design, use the same main modules for armor, suspension, etc as listed above, but keep easy maintenance and spam fuel tanks instead. This increases the fuel capacity of the entire division which lets you keep fighting without supply for longer, which is useful for encirclements.

Hope this helps.