Square taper installation and failure is not an orthodox issue. https://www.sheldonbrown.com/brandt/installing-cranks.html
This piece, by Jobst Brandt, is the most helpful thing. However it does not -quite- explain the critical solution.
Basically, >my left crank arm bolt loosened
this is not what happens.
What happens is your crank arm moves up the taper, and the bolt loses pre-load. The bolt is not loosening.
What you want to do when installing a square taper crank arm, is install it 'very tight' (with a short 3/8" wrench), once. That drives it far onto the taper. And then, you want to re-tighten the bolt 'somewhat tight' after a few rides. That will re-instate the preload if the crank arm moves further up, or squirms, but it will not force the arm on further, or work to enlarge/damage the taper further.
If you genuinely thought your crank bolt was loosening (and this is not impossible), after the number of re-tightenings you were prepared to do (maybe 3-5 rides), you might install the crank bolt again with a mild loctite. Loctite would be utterly pointless before you had re-tightened a few times.
How shops install old square taper cranks on old bbs is utterly beyond me (not being able to re-tighten after rides)
The idea ITT that this is all inexplicable magic and should be left to a shop does not make sense. YES it is inexplicable magic, YES it does not totally make sense. But there ARE things to understand and that is as far as a shop might possibly be out infront of you, beyond having a touch for certain torques. >t. been extremely neurotic about square taper cranks for years and experimented and tried to do my best and replaced a dozen square taper bbs and serviced more and NEVER had one fail.