Hey Dan,
as long as you use the Makita hand trim router, you have to use these proprietary collets that come with them. Just as the router itself, this has many disadvantages:
If you want to have ER collets, you will have to switch to using a spindle instead of the hand trim router. This has not only the advantage that it is a different type of motor, but also avoids using the hand trim router which is not allowed for use in a stationary machine (as per the manual – and Makita confirmed that as soon that you mount the hand router into a CNC machine, its warranty is lost) and by the type of motor that it is, gets hot and can start to burn if overloaded. And using a surfacing bit or any other bit larger than 1/2" is already a mechanical overloading – it’s always when using large surfacing bits that people make their Makita to burn [1]. You can find many reports of Makita burning in this forum [2], [3]
Spindles unfortunately require frequency-variable three-phase electricity which means, you need a VFD. However, spindles are induction motors, which are the workhorse in industry, with their high efficiency and longevity. See also Comparision of router to spindle.
ER collets are also found on what I call Best milling motor solution for hobbyists and semiprofessionals, which would mean being able to use so-called 43 mm “Euro” mount milling motors that are universal motors that don’t need a VFD, but have ER collets, an all-steel motor flange, and are approved for use in a CNC, like AMB (former Kress), the Suhner, and the Mafell. They have a speed control interface too, which allows their speed to be controlled by g-code.
Unfortunately you can not (yet) mount such a motor into the Onefinity at the moment, but it is a feature requested here.