Your RD-M591 is a 9 speed rear derailleur, and has these ratings:
High sprocket Max
High sprocket Min
So nominally your high gears are not high enough, there's a reasonable chance it would work anyway. It could be a bit noisier and have slower shifting in the high/hard gears.
Suntour Superbe Tech was actually a fantastic derailleur, even though there are write ups about how bad they were, not sure where that stuff comes from because I used one for over 150,000 MILES and never had one problem, I have another bike with the Tech on it as well and it has maybe 25,000 miles on it with no problems.
When the Tech is set up correctly ...
Yes, the chain should be able to clear the cage and the pull ratio is the same.
There could be also incompatibility in the ability to wrap a chain of certain gear ranges and in maximum and minimum sprocket sizes. M410s were available as GS (medium cage) or SGS (long cage), T3000 is SGS (long cage). It is not clear which one you had, but you should be fine ...
No it won't work.
The RD-M5120 uses the "MTB" 10/11 speed cable pull ratio.
Your 105 uses the traditional pull ratio, the one that was used for "MTB" 6-9 speeds and for "road" 6-10 speeds (exluding Tiagra 4700 and GRX).
The problem here is that it might be tricky to find a rear derailleur with clutch and traditional pull ratio. ...
No, MTB and road rear mechs and their shifters use different cable pull ratios (at 10 speeds and higher). A road 10-speed shifter will not be able to control an MTB derailleur properly. There are certain convertors available, although they are not very cheap (for how simple devices they are / e.g. J-tek et al.).
I'm surprised your cable has lasted that long. I get about 5000km out of a rear inner in 3x9 in an area with plenty of steep but low hills. Changing the inner would be a good idea at the very least.
But if the bike shop is servicing the bike regularly they should have already changed the cables, or at least suggested it.
At the very least note which way ...
Your shift cables need to be replaced, like NOW. This is a very possible symptom of deteriorating cables (as in, starting to fray and eventually snap), which makes sense considering 10 and 11s brifters have a reputation for eating shift cables. Juhist has a good answer explaining the mechanism and reasoning.
30Mm is a lot of riding and the cables are well ...
The most likely causes are cable friction and derailleur hanger misalignment. Indexing adjustment has gotten more sensitive to both as the speed count has increased.
Derailleur hanger misalignment will tend to reduce the margin for error in the adjustment, and make it hard to avoid certain gears where there's some jumping or noise. When it's aligned, the ...
I don't have a new one in front of me, but yes that looks like the cable has sawn through due to the extreme exit angle.
If the barrel adjuster itself is affected, that is easy to replace. The plastic outer part of the RD-R7000 barrel adjuster is different and special looking compared to most Shimano RDs, but I believe the steel threaded adjuster itself ...