You know what we could do is give Seyrun an eighth servant played by yours truly.
The justification for this would rely on there being enough reserve prana to summon an eighth Servant, or a leftover Servant from a prior Grail War existing. The former is impossible for reasons of the plot and actions of the Second Owner that are as-of yet unknown, and the latter is too tightly intertwined with GM action for me to hand it over to a player, plus the advantage it grants is patently unfair.
There's actually not much of a mechanical problem with there being an eighth Servant. In theory, there's nothing wrong with me saying that when tweaking the Grail, an eighth class was added or something of that make. The issue, then, is the matter that if there was enough prana to accomplish that, it would contradict certain aspects of the Second Owner's actions and cause them to not make sense. That's also not getting into how annoying it would be to introduce a nonstandard class in the first place.
If you can think of another way to justify it, let me know. I won't reject things on face if there might be a viable option.
@KoLDon't use numbers. There's a reason I only had Circuits on ranks, and it's because whenever you try to put them in terms of numbers it fails horribly. There's bits of information that don't reconcile well with one another, approximating is possible but a headache, and Mahoyo's stuff doesn't do any favors towards that either. Just stick with the ranks and rest assured that you're so much better than everyone else and Rin's in turn much better than you but that doesn't matter since she was never born.