In the end though, you have to build & test to be 100% sure.
There are (also empirical, not theoretical) studies / papers on the topic, from the AES for example, that are freely available; the issue has been figured out for decades. The conclusion overall is that you should always use the maximum port size that you can possibly accomodate in a speaker box, because there will always be some compression. There are other concerns like port resonances (only relevant for full range boxes, not subwoofers), or cooling (only relevant for PA subs).. but generally, the larger the port, the better.
Flared ports reduce air turbulence at the port ends, which results in noise. So flared ports reduce noise. They do however hardly or not at all reduce compression, which is a result of an inadequate inner port cross-section area. Port noise and compression are / can be two different things.