tri, noise and dpcm are all lumped together as a unit, so the delta counter offset (which is controlled by Zxx and also modified by dpcm sample playback) affects all their volumes together, collectively. it matters less for noise, of course, since that at least has volume control.
if no dpcm is playing, there shouldn't be any changes in the delta counter at all, so you should just be able to reset it to Z00 (which is max volume) and it'll stay at max volume. nothing else affects it besides Zxx commands. but if dpcm is playing it's always gonna be shifting some amount. famitracker has an option to force it to reset to Z00 on every note cut, but of course you'll get that little pop/click noise from the DC offset when you do. your best bet to keep volume high is to reset to Z00 whenever you can - if you really dislike the click noise, try to insert it on beats where new notes are happening, then it kinda blends in as a "transient" - or try leading into it with an interpolated run of Zxx commands from a middle level back to 00 so the pops will be individually quieter (the bigger the jump the louder the pop)