Waitroda,
How hot do your bearings get? I did not measure mine but I estimate over 60-70 deg C as I could not keep my fingers on the housing. Much higher and I think that the grease would start to smoke!

I have a plan of action on my spindle:

Bin the cheepo bearings and go for some middle of the road koyo bearings (still not expensive at 10 quid each). I could have gone with Timkens at twice the price but I think these should suffice.

Replace the rubber sealed lower with an aluminium labyrinth with a couple of thou clearance over the shaft.

Grease with short fibre grease (I cant afford the kluber grease and I think a decent bearing grease should suffice at the speeds that I am looking at)

Run it in a more managed way.

Possibly look at increasing the speed of the spindle up to 4500rpm in future if 3500 proves successful

I will update my build log to show the progress.