Some thoughts -

Skip a tub & lengthen shower another 4 - 20”? As a potential buyer, I would view the lack of a tub in the master negatively. Would you use a nice soaker tub if you had one?

We were thinking of doing a frameless glass shower with glass on the 2 sides going all way down to the curb.

Enlarge the shower to the window, this will allow enough space for an in-shower seat/bench if you want one. Yes, frameless is the way to go.

Put in an oval 60-66" freestanding oval tub? If yes, should we put tub diagonally in front of Windows? Or should we line it up against wall next to the shower and use open space next to vanity for a bench, etc?

Shoot for a 66” tub (60” isn’t long enough for most adults to soak comfortably). A rectangular drop-in would look nice placed against the left wall (enough room??)

Keep current big mirror or frame it or put 2 smaller mirrors up? It is an open high ceiling so I like the feeling the big mirror adds
Keep same vanity & just change hardware? Or change out vanity also? Will you post another pic of the vanity area?