I switched from an Etrex a few years ago but I never found the backlight to be a significant drain on the battery, although I only had it on the lowest illumination setting.
I measured it at one point. I've forgotten the numbers, but it was something like an extra 30% with the backlight on full, and an extra not-a-lot with it on minimum. Where it really makes a difference is when the battery is nearly depleted, as you can eke out a bit more capacity if you keep the current lower.
The problem with the eTrex is that the mini-USB connector was designed for infrequent desktop use like downloading photos from digital cameras or uploading routes to GPS receivers, and wasn't built to withstand the number of mating cycles that micro-B and USB-C (which appeared after USB charging became more common) were. The later connectors put the springy part in the plug, so when it wears out you only have to replace a cable, whereas with mini it's in the socket, so when that becomes intermittent it's a real pain.
TBH, I'd avoid using any USB connector on a moving bike if I had the choice. The great thing about the eTrex, compared to the Edge series, is that removable standard batteries mean you don't have to.