Did you try WDT to make every effort to distribute evenly? If you move the ground coffee around with your fingers, it may seem level, but there may still be density variations underneath. Stir thoroughly and tap the portafilter on the counter; that should settle the ground coffee in a homogeneous fashion and reduce tendency for channeling. Tamp once and go.

Also I second previous comments to fix dose and yield and only change one variable at the time.

Thanks all. I have done the variables at one time, not adding more to the equation. I have gone down as low as 15.8g in the past, but pure gushers, but am willing to try it again--but I also have to figure out what micro and macro to put my Vario at since all that works 'hand in hand'. Baratza recalibrated the one they sent me, where anything finer than 3J is a gusher no matter what on solid Medium roasts, and actually had to use a 4H with 5 seconds to first drip and 16g and shut off at 25 secs on a Medium Dark roast . The second double after that was all the same but 20 sec. to first drip with no retention of grinds, waiting with my Gaggia Classic for at least 5 minutes for heat to build back up. Sometimes it seems no rhyme to the reason. Baratza recalibrated because no matter what I did, I couldn't go beyond 1P without gushers, and I had recalibrated. I was advised to updose & go coarser together by numerous people & googling. So I did that & had a 5 second shot, but blonding at about 20 seconds. This was where I went down from 3J to 3L and updosed from 17.4 to 18.4g. So maybe go all the way to 15.4g and keep it on 3J? This is indeed a science project. Also, I am totally happy with a shot that doesn't have the first drip anywhere from 5-10seconds. I don't think I'm ever going to get the best shot in the world as living in a 1929 year old home with no a/c--subject to all of that side of the science project. Humidity wreaks havoc on espresso making, but that's a whole different ball of wax...
I noticed that some beans are just more user friendly also than others. At one point I tried to find someone to come to the house & work with my equipment. No one was willing for whatever their reasons. Now with COVID, that would be a big no anyway. The weird thing is, my drinks are still better tasting to me than almost every cafe I've gotten them in. Either I do better than I give myself credit for or baristas are having the same kind of issues I do.

Are you tamping with a decent bit of pressure? I couldn't tell from the initial post. I would assume a firm tamp would result in a slower pour - the grind being equal. I follow the 20 gr in 30 gr out "recipe" and am pretty happy with the result. YMMV!
BTW I think the Baratza and most grinder settings are relative - so not really comparable form one grinder to another.

I don't tamp as hard as I used to, just a firm, but not overly firm.

Possibly too fresh of beans today (3 day post roast), first shot, 15.8g, 3J, first drip was at 56 seconds, totally choked. Second double, pushed it coarser to 3P, 15.6g---gusher at 2 seconds. I think I am going to quietly scream and recede into the background for now as there is no rhyme to the reason. Trust me, I was making very decent espresso for quite a while--so maybe I'll just take a step back for now as my Gaggia is heating properly (did the temp. test with styrofoam cup), and there are no retention of beans in Vario as I turned grinder on for a bit after dosing out. When changing settings, even put a few beans in the grinder as others have mentioned before my actual dosing. thanks all.

This seems like a textbook distribution issue. Degaulle asked if you had tried the WDT method. I'm not a fan myself, but I'm not having this issue. You should give it a try.

Jack