Thanks so much for everyone's warm welcome and advice.Hi and welcome.
There are no absolute right answers to your questions, but here are my thoughts.
As to your next sport coat, I'd go with a herringbone tweed in grey as I find it to be almost as versatile as - and a nice compliment to - the navy blazer. To be sure, if your wardrobe leans heavily to earth tones, you might prefer a brown or tan herringbone tweed, but the grey really covers a lot of ground.
I'm tall and thin and, like you, prefer a more fitted look than the classic sack cut as the sack can look too big on me. @TKI67 is correct that part of it is getting used to a different fit, but still, my frame (and maybe yours) is not the best for a sack.
What I've done is, first, always make sure the jacket fits properly in the shoulder; otherwise, it will never really look well tailored. Second, a good tailor can "trim" the silhouette of a sack a bit, so that might be enough for you. Hence, I'd be really careful about "sizing down" as the driver of your decision when purchasing a sport coat or suit jacket should be if the shoulders fit and if it's long enough as those things, basically, can't be adjusted.
Another approach, which I take because it greatly increases my options, is to go with any sport coat that has near natural shoulders and not worry about it being a sack. If most of its features are classic sport coat, then the darts don't bother me and I get a much better fit.
In our internet age, until you find the manufacturer and fit that works for you, I suggest, where possible, going in person as stores like Press, at least the NYC one, will honestly tell you if something can be altered to fit. Of course, you can always order online, take it to a trusted tailor and return if he says it's a no go.
I know in years past, Press has had a trim fit sport coat, but I didn't see it in this year's sport coat offerings. If you want to do something inexpensive just to get started, J.Crew offers some nice natural shoulder (but darted and double vented) sport coats for very reasonable prices (especially on one of its regular sales). I own several and, while they are not as nice as the Press, Polo or O'Connell's ones, for example, for the money, they are great "knock around" sport coats.
Good luck, glad you've joined our forum.
We must have very similar frames based on your comments. I tried the J Press trim fit 36 and it was too narrow in the shoulders. The 38 trim fit was great in the shoulders but the non-darted sack part was too big. I had a tailor narrow the body down a bit like you suggested and it now fits great. I wish J Press had a bigger supply of 38 trim fit...I think I'm going to have to continue to go with Suit Supply since they fit great without any need for tailoring. Anyone know if O'Connell's carries a trim fit? Otherwise, I think it's just J Press trim fit and Brook Brothers Milano cut. I tried the Fitzgerald but it was too big.