Acer palmatum usually transplant fairly easily I've moved some bigger ones with no problem, even with very reduced roots but have had an occasional upset where part of the trunk or the whole thing dies unexpectedly. I generally bare root maples to make them easier to handle. Shaking the soil off roots does not seem to worry AP transplants. 2 years ago I got 2 about this size that were hauled out of a car park by machine then dumped in the back of my daughter's tray truck and left for the day. She delivered them that evening so the roots were in air most of the day. I trimmed damaged roots and planted them and both are now fine except for some trunk damage caused by the digger,
Your plan to trench is probably good insurance as new roots within the transplant zone will give added surety.
Just make sure you trench close enough to make any new roots actually in the area you will be transplanting. Too many people overestimate and trench too far out then have to cut off the new roots when digging the tree or find the root system won't fit in a pot. I'd be cutting surface roots to around 1 trunk diameter from the actual trunk for a transplant like this. Cut some of the longer branches back if you think the top is too far out of balance with the roots after transplant but I find that transplanted trees tend to self regulate.
I guess if you have some experience you are already aware that collected trees are actually much larger and heavier than they appear. I would estimate the tree itself to weigh around 15-20kg. Any soil on the roots will add to that.
Pots generally need to be quite a bit bigger than expected too. Allow pot diameter minimum of 5 trunk diameters which would make a pot size of 70cm - 1m diameter for that tree??
Zone 8 should not be too much colder than here. I think AP should be hardy at those temps so should not need any added protection but if it makes you happy then go ahead. I would be a little wary of sphagnum around the roots. AP are prone to fungal problems when roots stay too wet. Just water more if it gets dry in summer.
