Read here before doing anything:
Traditional reinforcement of the joints was done with a sheet metal dutchman on the inside face of the joint. If you do it on the outside face of the joint, it rots out. Gluing a tenon in isn't as bad, but it's still more prone to rot than the original pegged construction.
You should have multiple options for fixing the joints - consolidate them with epoxy without disassembling, disassemble and repair the pieces, then reassemble, tenons, dowels, and so on.
A lot depends on whether you want them to be truly restored and able to be taken apart easily in the future or not. I wouldn't worry too much about the strength of a through tenon - as long as it's pulled tight it's plenty strong.