But the new Bosch one I referred to does extract the dust and still clogs in no time.
Couple of suggestions as it does seem you're using it wrong in some way, as power sanders are a huge timesaver if doing more than a tiny bit of sanding of anything bigger than a doll's house
1: if sanding paint or varnish, too high a speed and / or too much pressure can melt the paint which gums up the sandpaper very quickly
2 Using the right type of sandpaper for the job. Don't try and sand paint with abrasive for sanding wood, as it clogs up and works badly. Likewise abrasives for paintwork are less effective for woodwork. Proper manufacturers have half a dozen flavours of abrasive for different needs, plus multiple grits for each.
Same with angle grinder discs - metal cutting discs are hopeless on stone and vice versa
3 dust extraction helps, ideally a hoover that plugs into the tool. The built in dust bag versions are only marginally helpful. A power outlet on the hoover that the sander plugs into is ideal as the hoover fires up when you turn the sander on but it isn't essential