Hey David,
if you buy a spindle and VFD, if you take care to buy an Omron MX2 (PDF Manual) or a Hitachi WJ200 (PDF Manual) (which are practically nearly identical), you can override the ModBus control by the CNC controller at any time by a switch wired to the VFD and pause the spindle after having paused your g-code program in the Onefinity Controller, e.g. over night, and switch your spindle back on prior to resuming your program next day. This is described here and here. Note that this does not work with cheap chinese VFDs! With these, you would have to run the spindle all night long / during the entire pause as you cannot override ModBus with them, nor can you switch the VFD off during pause, because when switching it back on, it would have no run / speed set prior to you resuming your g-code program (however unlike the hand trim router, a spindle is so silent that it would not be a problem with noise if it runs all night long with no mechanical load – just in case you want to use cheap chinese stuff)
Regarding pausing a g-code program, unfortunately, the Onefinity and the Buildbotics.com controllers can only finish executing the last command when paused, and can only execute the next command when resumed. Going back in the program after a pause and continuing at an earlier point, as with industrial CNCs, is not supported. However, you could do this with a partial G-code where you manually cut off the part at the front you want to omit.
Regarding noise, you know that a spindle is much, much less noisy than the hand trim router. Are you in an apartment house?
Welcome to the forum!