I would use the solution I used to put drain plugs in BTR auto transmissions sumps. Essentially brass fitting secured with flat nuts, washers and some master pipe sealant as pictured in my post here:
https://fordforums.com.au/showthread.php?t=11487042
Suitably sized reducing bushing e.g.
https://www.ebay.com.au/itm/Hex-Redu...0AAOSwk-1Z6x7d held with a brass flat nut like this
https://www.bunnings.com.au/kinetic-...k-nut_p4700216 and copper washers and plugged with solid brass plug. Of course the sump has to come off for this solution. I have several nutsert and rivnuts sets of various brands and quality. They are good for attaching odd bits of trim and fastening small hinges etc on older cars but I would never have thought of using one for a sump nut solution and I am not sure, that, as they are just a friction fit fastener, I would trust one (in fact I wouldn't) to hold especially somewhere subject to heating up and cooling down.