using a different color thread for both stitching and buttonholes - e.g. red thread with an otherwise vanilla blue dress shirt.
Not exactly, but close. Judge for yourself:
get horizontal buttonholes down the front of the shirt
Weakens the center. Tends to "gap". Ineffective in that it is very difficult to see buttonholes when buttoned. Even harder if they're behind a tie.
buy a bit of extra cloth to have save to have the collar and cuffs replaced later or have several sets of soft detachable collars and cuffs made in the same fabric as the shirt itself, rather than in a contrasting white
It is better to have the original shirtmaker make two sets of normal collar and cuffs and not attach one to the shirt. That way you'll insure that your correct pattern is used. Just store the other set away, not in the Sun. When you're ready, virtually any decent sewer can remove and replace. No so for actually making a new collar.
All of this is assuming that I could find someone who would do it - probably for a fee - go through the hassle.
Nothing you have said is a hassle. All are easily accomplished by talented seamstresses/ers. Of course, you meant
additional fee?
using a different color thread for both stitching and buttonholes - e.g. red thread with an otherwise vanilla blue dress shirt. 