Hey Miles,
I don’t know what exactly would be the price margin to see it as available for hobbyists (in rich countries hobbyists buy expensive “toys”). These STC Series Automatic Tool Change Adapters can be retrofit to a non-ATC spindle. It’s not under $1000 but they are far under the price for the Professional ATC Spindles of the same manufacturer (STC Promo video, More videos: mechatron+stc+youtube.com). They may fit spindles from other manufacturers but one should be aware that with the Onefinity 65 mm mount, the the spindle will not clear the stepper so it won’t manage the extra length. With the optional 80 mm spindle mount , the spindle clears the stepper so you don’t have this limitation, but usually these spindle/STC units do have two mounts. I have no idea whether the single mount of the Onefinity will be enough to hold it. Maybe one could mount a second 80 mm spindle mount on top of the other, but you loose Z travel length then.
Besides this manufacturer, I have seen that here some other Automatic Tool Changers are offered but I don’t know anyhing about them