I wouldn't venture to point you towards a particular holder, Nico, but here are a few general comments.
The basic Speedball holder has a couple of disadvantages to be sure, but I've never considered it uncomfortable. Rather the reverse in fact. The shape seems to be conservatively based on old designs, and I doubt that changing to a different holder will make a great deal of difference to the tired hands problem. Sure you're not gripping it too tightly?
One thing which might make a difference is a chunkier holder like the Carrot: I'm not personally convinced but there's a good deal of anecdotal evidence that fat pens help arthritis sufferers so maybe there's something in it. You may care to take a look at the
Selecting a holder page at Yoke Pens.
The main difference between long and short holders is that the former are more susceptible to accidental damage in my opinion. Obviously if a long tail is out of balance that's a problem, but it's unlikely that'll be the case. I made myself a 12" holder a while ago expecting to either love or loathe it, but it's much the same as any other in terms of handling, and it isn't even distracting. So you may as well go with whichever appeals more.