If they can get Dragon Quest XI on Switch, KH3 should be doable.
Yeah they may have to specifically redo some graphical assets and obviously no way is it running at 1080p or even 720p in portable mode, but it should be doable.
KH3 doesn't strike me as a game that's exactly pushing even the XBox One to its max.
960x540 resolution in undocked mode
1280x720 resolution in docked mode
No anti-aliasing + reduced lighting/shadow effects + maybe some reworking on environments should work. The cartoony style makes it easier to hide things and use lower poly models for example.