Anemia and dehydration does cause fatigue and your description of light headiness when standing is normally associated with dehydration.
I suffered from similar symptoms as yourself although never dizzy or lightheaded, nor to I get the eye floaters.
I believe a salt deficiency could be a cause of our issues and specialist J pouch Doctors dont give this the attention it deserves and family Doctors don't appear to understand or even acknowledge it.
Prior to my takedown, I dehydrated to such an extent I was admitted into Hospital twice per month nearly every month for six months.
At the time, it was due to my loop ileostomy, I wasn't absorbing a sufficient amount of fluid despite what I was drinking.
However, all the strange effects that occurred, such as the irregular hearthbeat, memory issues, talking giberish, muscle cramps all down my back and legs and the involuntary, uncontrollable arm shaking, was all due to low potassium, and it's this essential electrolyte that enables our brain to function, as well as regulate the heart: so who knows what other strange symptoms such a deficiency could cause.
I've noticed, that should I eat a salty snack, such as crisps (Uk equivlent to Pringles) my energy levels improve, not instantly noticeable but there's a moment of realisation some hours later.
I've mentioned this to my Doctor, but it has been dismissed as a sugar rush; however, I do believe eating the Salt flavour snacks makes a huge difference.
The large intestine would normally extract salts and fluid from its contents, especially during our sleep; we no longer have a large intestine; therefore, we're not absorbing the essential level of salt from our diet.
During an episode of dehydration, I was advised to ensure I urinate no less than 3 times per day and it should be of a pale straw colour; however, I've noticed my urine can be totally clear, therefore and technically, I'm hydrated, yet I'm still experiencing the feelings of fatigue etc, thus, I'm lacking the essential electrolytes, especially potassium.