I would suggest that if you are pulling the instrument cluster anyway, replace all the dash light bulbs and bulb holders with new.

Temp gauge is not a fast mover, none of the gauges really are. If it pegs with the wire grounded but otherwise doesn't work sender issue.

If you have access to resistors you can test the gauges.

74 ohms = low
23 ohms = mid scale
10 ohms = high/full

One end to ground, other end to sending wire. Note they are not super precise either.

Same resistances for all three gauges.