I'm surprised you call them pricey. How much were the ones you looked at? Over here (on ebay.co.uk) you can get 25 steel M6 rivnuts delivered for about £6 (and like I said, you don't need the insertion tool, just a screw & a nut). As the packs get smaller the delivery cost starts to dominate, so a pack of 10 is £4. Direct from a fastener supplier the
M6 steel ones are £0.12 each (plus delivery of course).
They're really easy to use & very strong. For your job, you'd drill a hole in the bracket, put a nut on a screw & then the rivnut onto the screw, push the rivnut into the hole & then hold the screw steady while tightening the nut. That's it: remove the screw & you're done.
I'd probably leave the body of the rivnut on the inside of the L-shape of the bracket (i.e. insert from the outside).
Of course if you've got a welder (you mentioned tack welding) then another option is so-called "cylindrical coupling nuts" (
something like this). With those, you drill a hole, insert the nut & weld it all together. That'll be stronger but you'll have to get rid of the black finish on the bracket for a good weld & then it may need recoating to stop it rusting.