You think the G400/G400s Sensor is still good for now days standards?
Pretty good. The 3090 is probably my favorite sensor and the 3095 is just a 3090 with Logitech's SROM and it's a great implementation. If you're actually looking to buy one don't settle for an MX518 and don't get a G400 with a PID less than LZ13333, those have angle snapping. I've seen some reports of the G400s having QC issues with the DPI buttons where they click but don't want to register, but I don't know how prevalent it was.
Native steps on G400 are 400/800/1800/3600
Native steps on G400s are 400/800/2000/4000
Being 309X architecture you'd want to stick to the lower steps. I would actually wait to see if the G403 Prodigy that got leaked is a proper G400s shape without too much nonsense, but that's just me
mx500 / mx518 / g400 are still solid mice imo
unfortunately the coating of the mx500 / mx518v1 isn't so durable
Luckily mine are V2s, one at the office and one at home since 2011, glossy coating still sticks to my hands like glue. The Logitech logo looks like someone sand blasted it at this point and they needed the feet replaced, but otherwise besides being a tad on the heavy side, they still work great.
One thing I didn't realize is that I've been using an interpolated DPI (1200) for years. Trying to go from 1200dpi*3.33sens to 800*5 isn't comfortable. Small movements seem to be a lot wilder and over-corrections abound. Screw it until the G Pro comes, no reason to relearn it with this shape now.