You can't use that rim and add heavy spokes to get a stronger wheel.
The
correct tension for 12g spokes is over what mountain bike parts are made to take. Hubs or Rims.
Any gauge spokes in any wheel are under no more tension than the rim and the hub flange can support. So if you overbuild the spokes nothing changes. The maximum tension is the same.
The different gauge spokes will behave differently at that same tension though..
The Halo SAS rim, for example, is rated at 135kg per spoke. The reason for a thinner spoke
wheel being stronger at 134kg is the thinner 14g straight section of the spoke is
lengthened more when it's under 134kg than a 12g spoke would be, so it can
spring back further than a 12g can, therefore the spoke that's at the bottom of the wheel stays in tension when a bigger bump is encountered causing the rim to flex.
The spokes at the top of the wheel stretch too and provide suspension.
A spoked wheel is a suspension bridge!
But for mounting a sheave or a rag joint there is the sideways load that is not at all what spokes were supposed to be used for. It definitely changes the picture.
Perhaps only some of the spokes need to be 12g, then? Only the ones that support the sheave need to resist the bending from the sideways load on a point on the straight section.
Then the rest can be thinner and therefore stronger. Not because they are tighter but because they lengthened more at the
same tension. You would have to figure out getting the rim centred, that should be fun.
It depends on the spoke tension rating of the rim, so upgrading the rim to one that will support a high tension is the first step to a stronger spoked wheel.
The same rim will also happen to be very rigid too, so it really is a good place to start with upgrading wheels.
Whatever tension is the maximum according to the rim manufacturer is all you can ever use regardless of the spoke thickness. Yet sometimes that information is
not instantly available and included in every advert.
Upgrading to a motorcycle rim is necessary to
properly tension and lengthen 12g spokes. They match. But the motorcycle rims are too heavy and the heavy spokes will give no suspension on a lightweight vehicle, performance will suffer from the massive wheels, and your bicycle hubs will be a weak link.
The sheave is a different matter and I wouldn't know about it!
It looks like structural abuse of a spoked wheel to my mind lol.
I think you can put 12g in just for the sheave and tension them about the same as the others.