dddd

Great suggestions, the old front derailers thankfully have a good length of forward protrusion where the tips of the cage can be bent inward to better control a narrow chain (these were designed for use with 8mm wide chain!).

The Suntour V-GT rear derailer features adjustable cage pivot tension! One has to unscrew the stopper pin on the cage with a screwdriver, then unwind the cage enough to reposition the end of the spring one position tighter (under the big/heavy chrome cap with a 6mm hex socket in it). Then you re-secure the cap, wind the cage back up, and reinstall the stop pin (add grease while you're in there).

If you tighten the B-tension screw to effect greater chain tension, the shifting will become sadly unresponsive, and may increase the risk of throwing the chain into the spokes. Remember that these derailers were designed for laterally-stiff bushing-type chains, so the shifting response is already challenged just by using modern chain (which are much more flexible than even the bushingless Sedisport chain).

I really prefer using index-shift cable housing for ALL rear derailer use. It really improves responsiveness, especially with friction shifters!
A supply-chain problem arises though when a 4x5mm ferrule (not 4x6mm) is needed to fit modern 4mm shift housing and still fit into the narrower ~5mm housing stops on old frames (and even on old stem shifters and rear derailers).
Shift cable housing really always needs to be well-captured by a ferrule, or by a deep, squared-bottom, 4mm port in the component.
But brake cable housing can be run without any ferrule if it simply fits ok in whatever stop that it rests against.

I occasionally find 4x5mm metal ferrules, my Schwinns needed them, don't know where to find more though.
A stepped ferrule is often the next-best answer, each situation is different, these differences are sadly not the best feature of old bikes!