This not uncommon. No, I do not have your engines so have not run into this specifically.
What you describe is likely loose, corroded or poorly done connections. THe fact you did a bunch of this last year and it worked tells me you have more to do.
Check the terminal crimps for signs of corrosion. Sometimes a recrimp with a GOOD tool will clean things up enough , then spray as below.
Do not ignore the ignition/start switch itself as they do wear and can fail.
Use a DMM to read voltages from the ign sw. as you operate the starting circuit.
Often there is a small relay close by the ign. sw. that powers the guages.
The switch itself only supplies a very small current to that relay which then supplies a much larger current draw to the guages, buzzers. alarms. Check for that relay and check that it itself has good power connections and good voltage.
Any plugs? THey can be trouble as the contacts oxidize. Carefully separate them. THe pins can often be cleaned of light oxidation with a pencil eraser. If need be cut into smaller pieces. THe sockets will be cleaned enough by plugging unpluging once or twice once the pins are cleaned.
Spray both sides lightly with something like WD40 or Corrosion X which will hold further corrosion at bay.
Be carefull with the plugs as bent/broken pins are trouble, expensive to replace.
Make notes as you go, use tape strips if needed. If you just undo a whole bunch of wiring you may have a rats nest defying sorting it out later.
I also add a smear of something like NoAlox to excluded further oxidation as I have cleaned the terminals. Screw terminals can benefit from this greatly.
Each screw terminal needs to be checked for tightness and cleanliness.