I would have a ribeye.

Simmilar symptoms a couple of times now, but loads of things match that list. Including stress at work.

Bloodtests showed I was low/borderline B12. Wife has an eating disorder (she's vegetarian) and so by extension I eat an almost vegetarian diet.

Started iron tablets and found after about 2-3 weeks felt better. Kept meaning to go back and grab a 'post' blood test to see what difference the tablets were making to my bloods, but as always that's on the 'do it next week' list.

Firstly...I'm not a doctor.
N=1 experience.
Some (not all) of what you describe is familiar to me from the past 2 years on and off.
For me ? Really low iron identified by blood tests. Which in turn results in lower haemaglobin.
No idea how I got like that. Maybe stress from work travel and lack of sleep during those travels, a couple of long bouts of chest infection (which is chicken and egg in regards low iron !). Also wife and daughter are veggie so similar to what Duncan74 said, I tend to eat tmwhat they are having 85% of the time. But why after 50 years living and 20+ with Mrs ? No idea.
I also LOVE milk and cheese. Which can reduce iron absorption.but again no change.

It manifested itself at the worst point with finding I could ride the bike steady all day. 80- 100 milers with plenty of climbing (typical 100ft a mile average where I live) were fine when peddling steady in zone 2. But as soon as I tried to put any decent effort in or sprint, I'd get about 10-15 secs before blowing up and gasping for air. Then repeat. HR just wouldn't go past 130 or so st the worst point.
But daily activity? No issue. (hospital consultant subsequently said that's prob because of having a decent base aerobic fitness).

Edit. To add - the fix ? Well after some checks to.make sure there wasn't any underlying problems from internal bleeding, ulcers or cancer, then it was iron tablets @ a way higher dose than the DRA.
(But be careful and follow doctors- too much iron is really rearly bad for you too).
Though I'd been secretly hoping to be prescribed medium-rare rib eye as Dr Jim recommended. Washed down with Guinness..

Just stop...

You can chastise BobAjobb too.

I remember when you used to contribute to this forum rather than just troll.

"Application of the term troll is subjective. Some readers may characterize a post as trolling, while others may regard the same post as a legitimate contribution to the discussion, even if controversial. Like any pejorative term, it can be used as an ad hominem attack, suggesting a negative motivation".

I was really tired all of the time, had headaches and started having heart palpitations. I tried reducing my training load but it made no difference. I went to the Dr and my iron levels came out extremely low. I had cameras inserted in both ends, but they could not find any problems.

I was put on high dose iron tablets. It took 2 months before my iron level reached the minimum recommended range and another 2 months before I was sitting in the middle of the "normal" range. I now take a 50mg iron tablet daily which is enough to keep it in the normal range. My digestive system had problems when I was taking the high dose tablets, but copes OK with 50mg per day.

You contribute nothing with your one-liners. OP obviously made a conscious choice to exclude meat from the diet and you suggested meat.

OP: "Should I get Zipp or HED wheels? what are the pros and cons of each? *insert bunch of context and relevant info here*"
jim, probably: "get a coach"

I would never suggest that someone get a coach.

You get my point

Dude, just go get a B12 shot. If it's a deficiency you're feeling, it'll get fixed instantly, 100% every time

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Strava

Just checked and it's 50mg I'm taking daily. But also the muscle rippling thing - I also take magnesium on a night, which definitely stopped my restless legs and reduced the rippling / cramps. Recover better too. Those are the only two supplements I take and although I've occasionally tried other things (cod liver oil, glucosomine, etc) these are the two that I've noticed do correlate to how I'm feeling - good when taking, less good when not.