How do you find the handling/change of geometry with your 120mm fork or does it just feel as great as it looks
No complaints from me about handling, with 530 axle to crown, 2cm of spacers and flat bar the bike feels very stable. During several rides with slammed stem it became more responsive, akin to my previous 26" hardtail (almost same reach/stack) which I liked too. In case you want more saddle to bar drop, there's a lot of places to shave stack off left (reversed riser bar, -17/20 degree stem, reducing travel). Take what I say with a grain of salt, as I'm not that experienced of rider to notice small handling or geometry changes.
I asked Deng Fu and they replied the frame is specs for 100-110mm travel so Im abit concerd how a longer fork would affect geometry but also headset stress even though the headset angle wouldnt be affected much according to my calculations.
Given my previous experience with open mold dealers, support personell (Melodies, Jennies and so on) know shit about actual frames sans what instructions say. Several centimeters are nothing to bother about, just like you said even same travel forks from different manufacturers can vary that much, add travel adjustment to equation and you pretty much get into very vague limits of what frame can handle.
If you want to try more travel, I'd get my hands on adjustable fork in desired travel range and try out what's best for you by riding it in different setups.