Dear community
I'm really struggling to shake bitter notes from my espresso.
I do get other (nice) flavours through but always there is a persistent 'overarching' bitterness that I can't shake.
Compared to boutique espresso bars there seems to be some underlying issue I haven't uncovered stopping me from achieving the godshot...

Beans: Various including expensive boutique roasted. Varying roast levels including light city roasts (ruled out as cause)

Equipment: Vibiemme Domobar Double Boiler with PID, Mazzer Super Jolly, bottomless portafilter, balance.
Using double basket but I have single and triple also.

Parameters:
Temp set to 91-94 deg C (normally 93) with 13 deg C PID off set. Measured qualitatively with k-type temperature probe. I know I'm in the right ball park +/- a degree. I've played temperature up and down +/-2 deg to make sure not too hot nor cold. No significant impact...
Pump pumping at ~9.2-9.5 bar

Technique:
Weigh 16.5 - 17 g into double basket (plenty of clearance from shower head). Grind, distribute with paperclip, flatten. Tamp. Flush pump a few seconds, extract.
Played with tamp a bit. Typically not heavily tamping.

Extraction:
Beautiful, even, centered. Getting 30 mL in 20-30 sec. Always look great with naked portafilter!
Generally stop before blonding. First drops usually 8-12 seconds after pump engagement.

I'm very consistent in my shots and flavours but still too bitter. I've come to expect better given the capital investment and level of OCD I apply to my espresso.

I'd really appreciate thoughts on what next to try....

Your temperature is fairly close to what I run for my VBM. I'm able to get sour espresso after dropping the temperature enough. You might try brewing five shots in a row with each one two degrees cooler than the previous. After brewing them all taste them one at a time starting at one end then moving towards the other then go back the other way. With that much temperature variation you should be able to find the temperature setting that shifts to noticeably sour espresso.

This is how I find the optimal temperature for espresso with each of my different espresso machines. I'm amazed at the difference in setpoint that's required for my three machines and I've also seen substantial variation based on what others post about the setpoints they use.

Good advice! More structured than my approaches so far.
I have dropped the PID right down (89 deg C) and still wasn't getting sour, but then tried ramping up a degree or 2.
Also not tasting back to back, which is a good idea.

I'm wondering if I didn't get sour at 89 deg C, then perhaps its running hotter than my temp measurements suggested - it wasn't scace after all...(polystyrene cup + probe)

I'll try dramatically dropping the temp even more.

Have some good water to sip between tastings to cleanse the palate, and don't forget to take a sip before the first one as well.