Eachine QX95 with DSM receiver and Devo10

Different protocols mean different channel orders and different channel mids and end points. You should check in the Cleanflight configurator if your yaw channel is really at 1500. If you are off from the proper channel mid this will cause a constant drift on that channel. If your channel mid is off in the Cleanflight GUI you can adjust it in the channel settings of your Devo. Subtrim is what you are looking for.

Yes the channels were a little bit off and I used subtrim to correct them. They are now all centered to 1500.

However, I can't get the TX to use the full channel range from 1000 to 2000. Minimum range is 1039 .No matter what I try (scale, offset etc.), I doesn't go beyond this limit and it also never reaches the max range of 2000.

Should I leave this as it is, as long as all the channels are centered now?

This is a common problem. At the end of this page are some options for the rxrange command.

Just FIY, you can also fix the channel ordering problem in deviation. There's a page for reordering channels you can get to from the mixer page.