Welcome to BF! If I was in your position, I’d start doing road riding on the old MTB while I did a lot of window-shopping, brain-picking and info-gathering at all the area LBS’s. I’d use the time to figure out which was the best shop with the best+smartest people and then have them match me with a bike that was the best fit for me physically, my budget and my intended riding style and needs.
If you go that route... by the time you get your new road bike you should have logged enough road miles on your overweight, comparatively sluggy old MTB that your new road bike will seem like a featherweight supersonic jet, you’ll REALLY love it and road riding will be exhilarating! Good luck with whatever you do.