Nextruder on MKS3.5s
It should be possible to simply assemble the Nextruder and use with MKS3.5s, the connections on the Buddy board is there already.
RE: Nextruder on MKS3.5s
From my experience running 3 Mk4S and a Mk3.5, I can't help but wonder why you would consider downgrading your Mk3.5S....
My Mk3.5 is set and go. My Mk4S's need constant baby sitting with their "never perfect first layer". I'm one of those unfortunate ones affected by the nozzle issue that has been discussed for five months on GitHub. But obviously there are many other users whose printers don't show that issue, so take my rant with a grain of salt. But I think you should have an excellent reason why you would risk modifying a working printer unless you're really into tinkering.
Formerly known on this forum as @fuchsr -- until all hell broke loose with the forum software...