Wheel building for big timbers/fatties

I build wheels professionally.

Rim
Modern triangular shaped alloy rims are a lot stronger than the rims of old. The crème de la crème is HED, but you will get a lot more value out of a Pacenti SL23 (my go-to alloy rim for the road), or an H + Sons Archetype. Velocity's A23 is also worth a look; it's very good value for what it is.

All of these rims are 23mm wide, so will result in a better handling, more comfortable wheel, especially when matched with a 25mm tire (or up to 28 if your frame can accommodate it).

I'd avoid Mavic. The rims are old technology and Mavic haven't invested in redesigns in years (they want you to use their full wheel solutions, sadly).

Spokes & nipples
I'd recommend Sapim Force spokes. They're butted and highly resilient, but have a thicker neck so are less likely to break. Use the same for both sides; lighter spokes on the non-drive side are a little gimmicky. For nipples, just use brass: DON'T use fancy thread lock designs, something like Sapim's Polyax nipples are fine. Grease the threads in oil; it makes the building easier and when the oil dries out forms a kind of bond that resists unthreading.

Build
1) Use a 3-cross build. When measuring spoke length, ALWAYS round up to the next mm. This ensures the spoke is all the way through the nipple, so the entire nipple is fully used to tension the spoke.
2) Insert drive side spokes first. Insert the pushing spokes (clockwise) with the heads out, so they will be on the outside of the cross with the pulling spokes (this helps if your chain goes off the cassette into the spokes).
3) repeat with the non-drive side. Again, pushing spokes are heads out.
4) True up the drive side spokes first to get your vertical play dialed in (serious, ignore lateral truing and get the rim vertically aligned, just don't make the spokes too tense.
5) Then get the lateral trueness tensioning the non-drive side spokes. By the time the rim is centered the spoke tension should be close to proper tension.

Hope that helps. Good luck!

Big Al at Wheelcraft is your man, give him a ring.

Mavic Open pro or Ambrosio Evolution are go to rims for a tough build.

Ask a good wheel builder to build a decent hub like your Shimano, but 36 spoke, 3 cross pattern using quality spokes and brass nipples, into a tough rim like a Mavic Open Pro. If you want to do it yourself, its a great idea but ideally you want access to proper workshop wheel building tools like a jig, dishing tool and tension meter.

Perhaps a local shop with a good mechanic could advise and assist if they have the time?

Slightly higher spoke tension, and balanced tension across the wheel (ideally built with a tension gauge) to ensure its actually well built, properly pre-stressed and then then reworked after tire has been inflated to 100psi, should ensure a long life even for the "more robust" rider

I assume you're doing the build yourself (it sounds like it, but not 100% clear). If you haven't come across it already, Roger Musson's book has all the information you'll need to be able to build a strong wheel. It's the most down-to-earth set of wheelbuilding instructions I know of.

By "horizontally deforming", do you mean something approaching a "taco'ed" wheel? I'm certainly no expert, but from what I understand, even very strong wheels can fail under significant sideways load. So I wouldn't necessarily blame the rim. I'll be interested to see what comments others with more experience than I have.

Try "Strong" spokes, 3-cross, make sure they're properly up to tension, should be fine I would have thought?