I own a BES920XL and have trouble getting my espresso to taste the way that I want it to (the way it tastes when I order the same blend from the cafe).
I have been through several grinders: Vario, Ceado e37s, and now the Sette 270.
The beans I use are fresh, usually medium roast, some blends, some single origin. In either case they come out thin and sour.

Yes- I know, I know. That means its underextracting. But even if I adjust the grind, tamp, temp, rdt, wdt, etc it comes out the same way. I've had shots from 18g in the basket which poured for 60 seconds and and 40gram out be thin and sour. I've had 18g shots which yielded 36g in 25 seconds be bland and sour. It just doesn't matter what I do.

So if I ruled everything out, all that is left is something in the machine. I've noticed the change happen after I descaled the machine the first time about a year ago. I've played around with the OPV valve and set the pressure to 8 bar (with coffee, not blind) and set the pressure to 11 bar. I wouldn't say one was better than the other, but it got me thinking about the flow rate. Currently it's at 10 bar blind, and without any portafilter it yields about 300g in 30s. Could it be the pump even if the built in gauge shows its fine? What's more interesting is the second shot comes out thinner and hotter than the first one.

Idk, guys/gals. pls help.

You can't always assume that because you adjusted the grind that the grinder is not the issue. You can grind find enough to choke the machine, but if it isn't a consistent grind you can get channeling which causes a thin extraction since only part of the puck is being extracted.

First question would be are you using a naked portafilter so you can rule out channeling?

Also, what kind of water are you using? That can have a dramatic impact.

Yes, I'm using a naked portafilter and a vst basket/ original breville basket. I can record a video if it helps.
I try to use 50% tap and 50% R/o. I've tried just tap and just R/O, too.

I'd experiment with the brew temp as well if you haven't already. Also, this thread helped me quite a bit to tame sour shots--

Espresso 101: How to Adjust Dose and Grind Setting by Taste

With a recent blend I ended up with a very small dose and extremely tight grind and that yielded a much sweeter shot. Hope this helps.

That's definitely worth a try. Thank you!

The easiest way for the community to help is if you can have someone video you going the whole way through your workflow starting at grinding the beans and ending with showing the puck after the extraction and show everything in between.