Lelit Mara "issues" - advice needed on potential fixes

Morning all,

I bought a Lelit Mara from here a couple of months back:

https://www.coffeeforums.co.uk/topic/56873-lelit-mara-under-12-months-old-%C2%A3600/?do=embed

First things up, I'm not a novice with brewing, I've been using espresso machines (Sage until now) for years. I'm fine with cleaning routines on this E61 (backflushing, lubrication etc).

Now since I moved from my last Sage machine to this, I've struggled to get a decent cup of espresso. I've got all my variables in place and I can end up pulling a shot with a Pullman/IMS basket with good flow, 1:2 ratio as per recipe and in about 35 seconds. Still doesn't taste right (and yes, I've varied time/yield either side of this and I get bitter or sourness, I cannot find the middle ground). I've burnt through about 3kg of coffee so far and feel like I'm no closer to a good cup from when I started.

I'm still feeling very frustrated with the process and almost at the point where I wish I had my Sage machine back.

Adamant that this wasn't all down to my technique, and having understood that the machine had never been descaled, I decided to take the top mushroom off to check - attached is what I found.

Understanding that the mushroom was almost entirely stripped of it's chrome, and the green areas were only prevalent on the remaining spots of chrome, I assumed that this was chromium oxide as it did not react in citric acid. The rest of the exposed brass cleaned up within seconds to a salmon pink colour, I cleaned it as best I could but some chrome still remains.

Further to this, I have noticed the following on the machine (might be issues, might not) - please can you help advise:

When making a shot recently, on pulling up the group lever the pressure gauge jumps (not gradual) straight to 4 bar, hovers and then gradually ramps to 9.5/10 bar after 6 seconds or so - every video I have seen of a Mara making a shot, the pre-infusion gradually ramps to 2 bar, then up to 4 bar, then up to 10 bar. I'm concerned that this is stopped me getting a good pre-infusion. Is this coffee-bound, aftermarket basket-bound or possibly machine-bound?
After brewing through the group, steam pressure is at around 1 - 1.5 bar. When steaming the milk, pressure on the gauge falls to as low as 0.5 bar. This doesn't seem normal to me. (I do having issues with the milk steaming where I can't get whatever milk foam I generate to incorporate well, but whether this is connected or not I do not know).
Any existing lubrication was brown/black when I dismantled the parts, after totally removing all gunk and re applying lubrication to cams/pins/joints, it turns black within a day or so.
Both steam and hot water wands are quite "graunchy" and creak heavily at certain points of movement of the ball joint, I've taken them apart and cleaned the parts individually and relubed but this hasn't fixed the issue.
The group lever still seems clunky even after decent lubrication.

I contacted BB (where originally from) but they are inundated with machine services so cannot help me extensively. They have suggested a descale to start, followed by possibly changing the seals and I'm fine with doing that, I'm just wondering a few things that you might be able to help with so that I don't bother them any further.

If I take part the E61 group, can I soak the entire pieces in citric acid or will it strip the finish from the outside of the group pieces?
Can I soak the cams and pins in citric acid too or will the descale through the machine be sufficient for this purpose?
Should I aim to strip the rest of the chrome off the mushroom (to prevent further flaking) and if so, how?
Following @DavecUK's guide to descaling, I can't see any reference to pushing descaling solution through the steam wand - is this correct or should I bring some descaler through that part too?
Is there anything else obvious that I should be doing to make this machine perform optimally?