I have 3pt forks for my B2601. It lifts a hell of a lot more on the rear. Forks are really inexpensive - mine fit both the rear 3ph, and the front (pin on quick attach). Definitely makes sense to try this first, and it could be really useful if it means you can leave your grapple on while you move things - less changing of implements.
I lift things with my 3pt quite high. Neighbours have a bank that I use as a loading bank. I can get things off delivery trucks with my forks, or off my trailer. Just takes a bit of thinking. Sounds like you have a large enough property you could easily build a loading ramp if you wanted. That would allow you to do a lot of things.
On the lift capacity of the L - it's only a little bit more to full height. But full height on an L is a lot more than full height on a B. You need to look at the loader curve - I believe the L loader will lift a lot more than the B to 'just off the ground'.
It sounds like the B is doing most of what you need. If it's only the totes, and if moving to 3ph forks solves that problem, then no new tractor needed.
In terms of calculating weight - my approach would be to work out what the B currently lifts. You say it lifts a half full tote. A full tote will weigh twice as much. If you want to lift a full tote, you need a tractor with twice the lift capacity (if it's working to the same % of its max capacity as the B is). Close enough. The L isn't twice. The MX is getting there, as is the Grand L. That's a lot of tractor to get down your trails though, so a few things would change.