voltage gauge bounces and lights flickering.

The main thing that causes this are things like bad connections / voltage drop in the harness as well as the ground side of things. Sounds like you may have fixed that part

Since you have already tried a lot of "stuff" I would next try the following. You can do this with minimal intrusion into the harness by doing the following

Get a "Bosch relay" and mounting socket, and an inline fuse. Not important, 10-20A or so

Get a spare VR connector pigtail, junkyard, etc. Ma used this fairly late, up to ?? MPEFI? or so

Disconnect your green field wire. Run a temp wire from the VR spare pigtail to the alternator field

Wire up the Relay. "Tap" into your switched "ignition run" to fire the relay. If you have a ballast resistor you can use that, and of course ground the other coil lead. Be sure to check for diodes in the relay by checking with your meter at the coil connections.

Wire the hot feed to the relay through an inline fuse to the starter relay terminal. Wire the load contact to your VR blue pigtail connector

See if you now have improvement!!!

Have you checked with a meter for ground problems? With engine running at fast idle / low cruise, make the following measurement with all load off, and again with loads turned on, headlights, heater, etc

With your meter on LOW DC volts, stab one probe onto the battery NEG post. Stab the remaining probe into the mounting flange of the VR. Be sure to stab through paint, rust, etc.

You are hoping for a very low reading, the lower the better, ZERO is perfect. More than .2V (2/10 of one volt) is too much and indicates a ground problem