Re: Only War!
Do you want the long version or the short version? Long version first, short version last.
Only way I see around the whole "Group Regiment Creation Process" that is part and parcel of Character Creation is for you to design the regiment to tell the kind of tale you want to tell, and to limit our choices for actual character archetypes by what seems reasonable. That being said, while a Tank Regiment may at first seem like a good idea for a semi-solo adventure it also puts a lot of strain on you to be the rest of the supporting cast, be it the rest of the tank crew or the infantry outside your tank which will inevitably follow up behind the armored push to hold the ground we take and to guard our flanks from anti-tank infantry. Regardless to your efforts however RPOL by it's very nature of Play-By-Post will limit our activity by the activity of the slowest poster in the group, as such one should not worry about player post rate or worry about allowing one player to post faster than another, but instead one should worry about GM post rate which will be bogged down heavily by the necessity of playing all supporting cast for each of our individual stories should you go down the semi-solo adventure style.
In short my suggestion is that you pick the Regiment and what Archetypes would work for said Regiment and the story you wish to tell, but don't become overly fixated on supporting a semi-solo adventure style because it will be an unnecessary stress on you the GM who has enough to worry about without interacting with us incessantly.
Now if you have trouble actually building the Regiment itself, don't fret, there are pre-built regiments available that would only require the tiniest bit of alteration to fit whatever style of Regiment you want them to be.