Sorry for not responding earlier, I'm around mornings lately.
Threading in advance is unnecessary. Extra work with no gain. You'll find the lengths vary slightly anyway depending on the direction spokes are entered into the hub along with irregularities. Some spokes may extend past the nipple heads more than others, or possibly not at all.
I suggest attempting to tension the wheel yourself with the hub mounted in a upside down frame. Rear hubs are too wide for a front fork. Attain the proper size spoke wrench for 12g nipples.
Cable ties can be useful.
View attachment 203387
A problem is that the ties can be rotated out of position which could cause the rim to shift off the center line of the frame. Keep flipping the wheel in the dropouts checking the wheel doesn't tend to one side. If it does, tighten or loosen the spokes of one hub flange.
Start tightening spokes in the area where the rim is furthest away from the hub. Easy to spot, where the rim jumps when rotated, called radial hop. Pulling the rim and hub closer to each other is a way of looking at tensioning a wheel.
Work the further points first to reduce hop. Truing laterally is easier. As you practice more you'll be able to true radially and laterally simultaneously. Work slowly with short turns per spoke going around. Use the valve hole as a reference point during the progression. Lastly, practice tightening spokes from different orientations so you're tightening/loosening correctly. It can be confusing for first timers.
Helpful hints for encouragement. . .
Search around YT for wheel building videos, unfortunately I rarely watch them to make a recommendation. Perhaps a member can.
What do you lose by trying? If you really can't, there's no shame taking it somewhere else. You might surprise yourself. You'll never get it if you don't do it yourself.
ciao