I usually know better than to post this type of question, but I am troubled by what I see and interested in ideas.

Reminder, low-rev lefty.

I have a spinner, but any time I sand a ball, any ball, it's toast for 2-3 games (IMHO until it gets back to lane shine). If I use a high grit, it slides past. If I use a low grit, it labors. In all cases, for all pressures, grits  and lane conditions (only bowling 2 places though), the results are always negative for the first few games. I know all the variables and their impact, but still......

Possible answers I've come up with are "you're always using too much pressure, so it's always too high until it gets back to lane shine"... things like that.

I don't know how else to express this...
Thoughts?

In my experiences with what I have seen; if you are low on revs, you tend to need more help on the backend of the lane, not up front unless you are speed dominate.  If you sand it too low, the ball will have no energy left on the backend and likely that is what you may be seeing.  Once the surface smooths itself out a pinch, that is when the ball comes back to life. 

I agree 100% on the compound idea.  Sand your ball to 2000 or whichever you are happy with and add some compound over the top of it.  The compound will help the ball conserve some energy for the backend without the ball wanting to go 45-50 feet in the process.  The underlying sanded surface will still do its job and the ball wont seem to be dead for the first few games.  Try that and see what happens.

few years north of 50 myself and LH
Ive had some luck with more underlying grit and then polish
Like 500 + polish or 1000 + polish
Even go to this polish at times - powerhouse clean n sheen
its a matte polish, provides some additional midlane traction in my experience
https://www.bowlingball.com/products/accessories/powerhouse/10702/clean-n-sheen-32oz.html?gclid=EAIaIQobChMIyNuQr4_15wIV0MDACh10fwoOEAAYASAAEgK3ZvD_BwE
(I'm sure there are others than provide similar qualities)

But we are all talking about the same thing that we want, matching up to a particular lane surface and oil pattern, even if its a house pattern.
When you have the wrong ball in your hand, you know it.
Definitely takes surface tweaks to fine tune what works for you
the 1500 dull is to much by your description of what happens early in the game play.
Keep trying different finishes until you find what works
good luck