Oh @Matthew54 I really feel for you on this, I've been there. I had 2 MCs, then a successful pregnancy, then another MC, and now pregnant (14 weeks so still holding my breath...).
The thing is that by changing your behaviour you are implicitly saying that you did something wrong last time, which you almost certainly did not. Most miscarriages, particularly first MCs (but actually, probably all three of mine) are due to genetic anomalies and no amount of eating right or supplements can prevent a MC in that case. So you're trying to assert control over something that is (a) not controllable and (b) not your fault.
That said, some first MCs will be followed by others with the same underlying and solvable cause so I think it is worth seeing if your doctor will run any blood tests (e.g. thyroid function, iron, Vitamin D, Vitamin B12, antiphospholipid antibodies (primary care often can't do this one alas)) or refer you for a pelvic ultrasound just to check things out. But if they won't do it, then remember the reason for that is that in the majority of cases no underlying cause is found.
I did make some changes between my MCs and successful pregnancies, and I tried to take a balanced approach to making changes I thought would help me vs. not. I added in ubiquinol to my supplements because to the extent there's any evidence about improving egg quality it seems to say ubiquinol (Co-Q10) might help, but that evidence is weak so meh, marginally worth it. Other than that I didn't go down the ISWTE rabbit hole for exactly the reasons you've described and because i thought there was a bit of risk with putting a bunch of dubious supplements into my body so wanted to limit that.
I also cut out alcohol and caffeine completely, but that was mostly because I thought it would help me manage my anxiety a bit better. I'm not a big drinker, but certainly can sometimes use an aperol spritz as a crutch... and I had a coffee a day which is well below the guidelines for early pregnancy (and I'm not at all convinced caffeine causes MCs anyway) but again was more for the sake of my anxiety.
So I guess my suggestion is (a) let go of the idea that you could have done something differently to prevent your MC; sadly, this is almost certainly not the case, (b) try and get whatever testing your GP will do to rule out systemic causes, (c) come up with 1-3 things you can do that you think will make you feel like you're doing something and like you can let yourself off the hook a bit;; oh and (d) be ruthless with the keyword blocking on social media - the algo feeds off stressing you out, so block early and often. And lastly, managing your mental health after MC is incredibly difficult. Be kind to yourself.
Long message but I hope a bit helpful!