I've personally done a little bit of theory crafting for the Protector synergy feat line and it is one of those weird ones that based on existing material published at the time you couldn't actually get the most mileage going purely Herald/Fighter given Fighter didn't have any tertiary casting archetypes at the time. While I love synergy feats in general, I feel that in most cases they tend to lean more into one class rather than effectively blending them in most cases. The Protector line leans more into granting an alternative means for the Herald to boost themselves with magic, but is ultimately still focused on being a warrior type first and foremost. Honestly, I think it would have been better suited as an Archetype rather than a synergy feat line. Or even just a feat line without having it restricted to classes, just have the requirements be something like "have Extra Attack and be a Spellcaster"
To get the most flavor out of it, I figured Cleric/Herald/Fighter would actually be best to get the most potential milage out of it. Adding a tertiary caster archetype for Fighter while nice, would just add an extra spell level which at that point isn't especially needed at the end game of the build.