I’m afraid the Carbide Compact Router tops out with a 3/8" collet (Makita makes one which fits: https://www.makitatools.com/products/details/763619-3 [9] https://www.toolmarts.com/makita-763619-3-3-8-collet-for-3608b [10] — supposed to be available from Elaire Corp. as well as other sizes.
Elaire has various sizes at: