Some basic general thoughts on the build process in this game. Curious if this makes sense to any other players here:-
One starts off a character with some basic sets that use the widely available ordinary masteries, sets like Shukuchi, Combat Training, Final Resistance, The Best Choice, etc., as appropriate to the class. Here one wants to have at least one good and appropriate defensive set, one offensive set, and if possible one tactical set (one of the sets that's conditional on the character's normal fighting circumstance - whether they're normally going to be surrounded by enemies or friends as they fight, etc.).
The point here is that even at this early stage one wants to have already some basic "shape" to the build that exploits what the class is best at (which is usually marked by the inbuilt mastery that defines the class).
Then as one harvests more of the class, sub-class, and human masteries, one moves on to the (usually more powerful) mastery sets that include them - but (here's the trick) one only replaces the more basic sets when one finds a more advanced set that does roughly the same, or a similar thing, so that one is still maintaining the basic "shape" that one started off with (having a build that exploits what the class is best at).
And THEN, so long as one maintains that balance of offensive, defensive and tactical sets, one "fills out" the build with "dangling" masteries (masteries that aren't contributing to any sets) that are independently powerful, that really stand out, or are appropriate to particular circumstances and enemy/environment types (Impulse Fields, Veteran, Snowman, etc.). At this point one should also pay increasing attention to Speed and Action Time (especially lengthening enemies' AT and shortening one's own -
especially if one is using a lot of responsive attacks - responsive fire options for ranged, or things like Counterattack and Forestallment for melee, all of which lengthen one's Action Time, which needs to be compensated for if one wants that character to do stuff more than once in any given combat!
).
As a general rule, I think that it's maybe a mistake to fill in all the advanced mastery sets one can possibly have just because one can, and completely ditch all the basic sets from one's build. Of course that's going to happen sometimes - I think particularly with the magic classes, they seem to benefit from having a lot of the advanced mastery sets. But one should try not to completely let go of ALL the basic sets, if at all possible, because some of them remain quite powerful all through the game (e.g. one's melee fighters should probably retain Combat Training until one really has something BETTER from the more advanced mastery sets, or some combination thereof).
This makes the build process really engaging, and the depth here is a speciality of this game. As with all good progression systems, one is constantly weighing options - is it REALLY worth replacing THIS bunch of stats, procs, conditionals, etc., with THAT? It's a constant ongoing process of "massaging" the build and then testing in-combat, as new masteries and sets are discovered. (In parallel one is also doing the same thing with gear, just as one would normally in most games - but I think one should prioritize the build, and have the gear fit around the build.) But the other interesting thing is that the appearance of new possible mastery sets is always tempting one away from the build one had, so there's a process there of weighing up whether wandering off-piste is going to be worth it.
Always maintain that basic "shape" - a general outline of a few sets that balance offense, defense and tactical, and exploit what the class is good at, what the signature inbuilt class/sub-class mastery does, etc. Then one can "fill in" with other good single masteries or interesting sets, to taste.