I wouls also take in account that the only transports that the 17cm and 21cm can utilise is the Sdkfz 7
https://panzercorps.fandom.com/wiki/SdKfz_7 (70 prestige),
while the 105 and 150 can have the Opel Blitz (50 prestige) or the Hanomag
https://panzercorps.fandom.com/wiki/SdKfz_251/1 (100 prestige) or 250/1 (200 prestige).
So when you upgrade a 105 or 150 you lose the prestige spent for the transport.
- With the Opel you can have a cheap combo
- With the Hanomag you can have a more offensive unit especially in the early years ( It's a combat unit, not merely a transport), since you can use it to finish off an enemy unit after moving 6 hexes forward, making it better suitable for a flanking or fast advancing army group.
IMHO the best use of the larger calibers is to let them
without a transport, entrenched, to defend. Or following a slow moving army group (all without transports)