The biggest problem with the Pixel 3a is that it would be a sidegrade relative to your G6 in most respects (and a downgrade in some others like build quality and screen resolution), and its drawbacks might get a bit long in the tooth over time. Other than the camera and the timely updates, the 3a has only 4GB of RAM, a pretty meh display, and a midrange processor, which might be fine for 2019 but not so much in a couple of years. And that chin, oof.
If you don't mind forking out the cash, your best bet is probably the Galaxy S10, since it's about the same size in hand as the G6, has a headphone jack and it's a tremendous upgrade in every respect over it. It's worth noting that this phone has the fingerprint reader under the display, not on the back like the others.
Another choice would be the LG G8 ThinkQ, with flagship specs, headphone jack, also same size as your G6, and a POLED display. It has a pretty big iPhonesque notch though, and I don't know if that'd be a dealbreaker for you (ït'd be for me). You could probably get one for less than $400.
If you don't want a notch and you don't want to spend as much money, then the Pixel 3a is fine.
Here's a handy size comparison.