Another option is https://www.transomrepair.net
although some grp purists think it's for amateurs.
Ply is relatively cheap, readily available and as long as you can seal it up properly it's bound to last as long as the boat will.
A thread I read a while ago about alternatives. Wood free Transom Core - The Hull Truth - Boating and Fishing Forum
Coosa board looks interesting and similar grp layup. Problem with solid core would be the weight, the cost would add up and you would have to laminate in stages to stop thermal build up when laying up.
Are you repairing in epoxy or polyester?