Based on lack of response, I'd say no-one has experience to share
This might be unsurprising. Subsequent findings on the internet on the subject - not much content admittedly - seem to agree that SLA batteries don't like deep discharge; it dramatically reduces their lifespan. OK for a PV system maybe, but not for our application.
Any advice, then, on what I should replace my US-125s with? A new set of the same, or should I undertake a big project and expense and go for LiFePO4 batteries (which seem to require a controller and motor upgrade)?