My David 2 has a sliding beater. If yours does, too, this may help. I remove the front beam and the whole apparatus with reed and rails for the sliding beater. Then I sit in a dining room chair (lower than my loom bench) just as close as I can get it to the loom to thread the heddles. I put an LED floor lamp shining on the heddles so I can see to thread them easily. And I take breaks to straighten up and stretch pretty often.
I hope this helps.