I'd think the limiting factor most mounts would be the seat stays on any given bike. Mounting to the seat frame could take some of the pressure off, but even so, 310 lbs seems wildly optimistic because don't forget: The whole bike has to support you, your cargo, and all the extra parts/bags. The wheels themselves also have a weight limit and with a loaded down bike with a bigger rider, you could easily get into 500lb+ territory.
The seatpost rack would probably be OK, just don't try to load it down with more than a fraction of that 310 lbs (if not for structural reasons, then for practical ones). Bike trailers are another option for expanding capacity without adding more wear and tear to the bike itself (just be extra careful with how you load it up, putting weight on the wrong spot can make handling increasingly difficult - the general rule is to place 60% of the weight in front of the axle and 40% behind which applies to bike trailers and vehicle trailers alike).
Another important thing to do is make sure you have good brakes: That's very important, but even more-so when you're using a cargo bike. If your rim brakes aren't up to snuff, consider upgrading the pags to Koolstop salmons, they're very well regarded and new pads aren't a big investment. Make sure that your braking system as a whole is up to snuff/in good repair because the more weight you carry, the sketchier stopping can get.
I'm late to the party but I use my Trek 820 as a combo commuter/grocery getter and have not had luck finding a front rack/basket that works with my front suspension (though I haven't really made a dedicated effort yet).
For now, I've settled on just using the rear rack with pannier bags combined with a saddlebag and a big frame bag.
It's not quite as versatile as my ebike with 50liter pannier bags, but I am still regularly impressed with how capable a cargo bike it is despite being an old singletrack MTB.
If you find yourself struggling with front cargo options, it might be worth swapping to some sort of touring fork which can support not only a front rack/basket, but bags on each side of the fork as well.
Good luck and I hope you'll keep us posted on your progress!
