MineColonies doesn't have much to offer for way of multiple colonies, but workarounds I've seen are, as you suggest, disowning a settlement or configuring the settlement size to something ridiculously high, then moving the Town Hall block once your Town Hall building reaches the level you plan on leaving it (level 5, being the highest, and taking some time to accomplish).
If you disown your settlement, I'm not sure they will continue to be productive, especially if you don't have something supplying the colony with required tools. If anything goes sideways, you may not have the ability to make the needed adjustments, since you no longer have a claim to power.
The challenge with the single large colony approach is AI behavior. Builders want to travel back and forth to their building to grab supplies, which is problematic if you're starting an outlier at great distance. The best workaround for this I've seen is to build a Builder's Hut at the new site first - place the block, hire a worker, then recall them to the new work zone. This should limit the travel and other derpiness since they look for all the supplies they need in the newly placed block. After that, you should be able to have that Builder work in the new zone to build the rest of your outlier.
Technically, you don't need to move the Town Hall block. The colony borders are defined from the first placement of the Town Hall block, and moving it after that first placement won't affect them. You could just as well build a mock Town Hall without moving the block, but if you intend for the builder to do the work, you will either need to move said block to the new site, or use the Scan Tool to provide them with a schematic. If you wanted to use one of the existing schematics for a Town Hall (desert sandstone motif, for instance) you're likely going to need to move the Town Hall block to do it.