Hey Mark,
a 1/4" T-track is 3/4" (19.05 mm) wide and 3/8" (9.5 mm) high.
Image: Jig and Fixture Tracks – Lee Valley Tools Ltd. Ottawa ON
If you take MDF slats of 3/4″ (19.05 mm) (or as Onefinity shows here, of 22/32″ (17.5 mm), then you see that you have plenty of MDF that you can resurface many times before you reach the T-Tracks one day. When you surface the wasteboard, you only take a thin layer away.
Just be careful with the hand trim router burning. Makita sells only bits with 1/2″ (12.7 mm) maximum for it, and they surely know why. As was reported here in the forum multiple times, it was often when using large surfacing bits of 1″ (25.4 mm) or even more that the hand trim router started to overheat and to smoke. This is because of the type of motor (see also differences between hand router and spindle)
Image: QCW Frame, view from the front
An example of what you can use in the slots is:
Leevalley.com Accessoires
Onefinity says