Trigger shifters get sluggish and sticky over time as the grease in them ages and cakes up. Especially that latch that catches onto the dial teeth as you thumb it. Most of the time it can be flushed out with can spray solvents, but I can rebuild them properly as I do it so often. Experience builds confidence here.

Grip twist shiffters are less problematic as they are all plastic, many without even any metal springs. When they are hard to turn, the problem isn't the shifter, it's wire in the cables and housing.

Same goes with thumb shifters, it's almost always just the cable and housing.

Can a novice DIY all the above?

Sure why not.

If they screw up and break stuff, can always downgrade to a pair of all-plastic thumb shifters

These actually work. I use many to repair dumpster finds.