Hey Don, hey all,
this diameter, 65 mm on Makita RT0702C, 68 mm on Dewalt DWP611 (used on another hobbyist CNC machine, not matching Onefinity machine) comes from the fact that these devices are hand trim routers, made for hand-held use:
Regarding spindles, which are induction motors as used in the industry (difference router/spindle), many are available in exactly these diameters to replace these hand trim routers, but 65 mm is rather narrow, so for water-cooled spindles, 80 mm is a diameter with more room for the water-cooling sleeve inside, so 80 mm is the most usual diameter for spindles up to 2.2 kW and ER-20 collets, so Onefinity met the multiple demands by offering an optional 80 mm spindle mount,
but many spindles have other diameters of 100 mm or more and most of them are not cylindric at all and require replacing the Onefinity Z assembly with a universal Z assembly, to be able to attach spindles like these, these or these (see also reputable spindle manufacturers), or VFD-less milling motors like these. The good thing is, the original Onefinity Z assembly can be removed easily and then you have a flat surface on the front of the X carriage which makes attaching an alternative Z assembly easy!