This one’s been suggested before, but I have a take on it that might be more amenable.
As it stands Spirit Guides can change points spent in the mastery skills but not the mastery bar itself. This makes experimenting with builds difficult and essentially locks a character into a few builds. I feel that the high level of builds that a character can have is part of the charm of the game, however.
While I understand that we should not be able to change a full mastery tree (otherwise one character could be anything), changing mastery bar points is sort of fundamental to experimentation. To that end, why not make the first point spent in a tree nonrefundable, but allow all other points in the tree to be changed by a Spirit Guide for the normal fee? This would still lock characters into a particular combination of 2 mastery trees, but would allow for experimentation and adaptability within that setup.
Just my thoughts but Titan Quest was the origin of the mastery bar system and dual mastery system. In that game the stat value alone of filling both of your masteries stat bar meant that every min maxed build would have full mastery bars even if the upper tier skills weren’t particularly important. So going to Grim Dawn I like that there is more diversity in that it is actually a choice how far to fill rather than a ‘no brainer’ to fill both bars by the middle of Legendary difficulty. So this problem you describe was kind of created in going to that level of choice and build diversity.
So with that in mind like you are saying we need a solution to be able to play with builds specs, but I think if you could just unspecified the mastery bar it might make less desire to plan out builds and replay.