If you go with Velocity, they have one page that will help you pick a rim. Tubless or not? Weight, light or heavy duty? Width, it lists the best tire width for each rim.
https://www.velocityusa.com/tech/rims/
I use Dyad on my light touring bike, I anticipate only using tires between 28 and 37mm width, and tube type only. The Dyad fits that criteria perfectly based on that page of data.
At the bottom of this webpage there is a table that shows best tire width for inner rim width if you use a different brand of rim. There are some people that say that table is too limited, but I find it works great.
https://www.sheldonbrown.com/tire-sizing.html
On my light touring bike I use 32 spoke front, 36 rear. On my medium and heavy touring bikes, 36 front and rear.
You asked about wheels, not rims. But I build my own wheels for my bikes, I have no clue what would serve you best for hubs, so I can't comment on that. And my favorite spokes (Wheelsmith) are no longer manufactured, so I can't suggest a spoke, but the last wheel I built used Sapim and they worked just fine. For the past decade I have used Sapim brass nipples.
If your Ortlieb panniers did not stay on the rack right, either you used the wrong spacing on where the hooks are on the rails, the wrong inserts for rack tubing diameter, or perhaps there is something wrong with your rack. But what you describe is not something that should happen with Ortlieb panniers on a good rack. Go to a store that sells Ortlieb panniers and ask a sales person how to adjust the panniers for the rack.
Some people have put a second lower hook on Ortlieb panniers but I have not found that necessary.
If your rack tubing is unusually large, for example a Tubus Tara, Ortlieb makes a hook with wider capacity that you can buy.
I put plastic tubing on my pannier racks to protect the metal from chafing, and the Tara tubing with that extra layer of plastic tubing is too big for the Ortlieb hooks. Instead of ordering wider hooks, I make some, used 1/8 X 3/4 inch Aluminum bar stock, put some inner tube rubber over the aluminum. Photo below.
Photo below is a blow up of the lower hook shown in the above photo.
