My daughter was given a 26" hardtail with a slightly weird setup to use as a utility/commuting bike by a guy whose hobby was to build up bikes from old but unused parts... the crankset & mechs are Deore M510 / M590 3x9, but the cassette is a 7-speed, with matching 7-speed shifter & 7-speed chain. I don't know how much difference it makes having the slightly fatter chain on a crankset designed for a 9-speed, but she's definitely never got the hang of a 3x system, crosschains like crazy and doesn't think to change down a chainring 'til she's standing hard on the pedals. The result has been some pretty dramatic failed shifts, jammed chain and broken teeth on the big chainring after about 200-300 miles.
Obviously I could swap to a 9-speed cassette, shifter and chain, replace the big chainring and whatever part of the problem is caused by the mixed system goes away. But I'm thinking this probably leaves the bigger problem, and changing it over to a 1x system might be a good idea.
If I do the latter, I would be thinking to put a narrow-wide chainring of suitable BCD on the middle position of the current triple crankset, remove the other two along with the front mech & shifter, and upgrade the rear mech to a clutched, 11-speed setup to give a very similar range of gears as the current 3x7 setup. It seems like the current FH-M756 hub will work with an 11-speed Shimano MTB cassette, but not 12-speed.
So the questions:
How big an issue is the current mix of 7- and 9- speed bits likely to have been? If I blame that for poor shifting, and believe she'll get the hang of the triple if it works smoothly, fixing the 3x9 saves ~30% compared to a change over to 1x11... the frame is entry-level, so I don't want to spend more than I need to on this bike.
Is there anything else I need to consider in converting to 1x... I assume the middle ring position is the best chainline option?