Automation?
Automation?
Thinking about the problem after viewing some of picures and your and archy's comments, it seems to me that by having breech loading mortars you could put the crew under armor, automate the cannon, and with the proper mount, use it for direct fire and low angle indirect fire as well as high angle indirect fire. In effect, it's no longer a self-propelled(SP) mortar. It's a SP howitzer.
Nothing wrong with that per se, except with the weight, which is the main part of the original problem, transporting it to the theatre of operations, and then having ease of mobility to keep up with the maneuver battalions so they can provide direct support.