If you're using a portable TS, a tech I've been using for decades at jobsites is to set 3 2X4s on horses, cantilever 2 2X4s past the horse enough to slide the table of the TS over them then place a sht of ply over the horses. If you don't have help you can set a couple rollers up fore and aft the table and TS to stabilize the length of the board.
For the long trim boards you'll need the straightest ones you can find at the lumber yd., on an existing structure odds are high the house has settled and moved, the winds may not be as lined up as they once were, the old trim made some subtle moves with it.
As for the trim angle take 2 pieces of 1X the same width a foot long place them on edge across the trim. Mark the inside and outside points where the 2 pieces cross, connect the marks with a line and you have the angle.
I would not butt the miters; I'd lap them for glue, nailing and or screws, I would expect overlap when assembled on the wall so prepare for some RAS or hand planing use to smooth the exterior lines.
I'd also consider holding the trim a 1/4" off the brick sill, I'd place ice/water barrier on the wall then flash it with a lead pan for runoff.
Work smart not hard!
Never bite the hand that looks dirty