Chronic tiredness can be such a hard thing to figure out, sorry you've been going through that! Is it sleepiness (as in can fall asleep at any point of the day), or fatigue (as in non-restorative sleep, brain fog, feeling exhausted, etc)? I've had chronic fatigue (non syndrome) for six years now, so I'm pretty familiar with how obnoxiously long it takes to get answers. Some things to look into:

Do a full polysomnogram with MSLT: this will check for sleep apnea, restless leg syndrome, narcolepsy, and idiopathic hypersomnia

Look into ME/CFS if you have post exertional malaise (pretty much you cross some invisible line in exertion followed by a delayed crash).

Look into MCAS if you also have strange allergy symptoms. I have MCAS, though tbh I didn't really have that many symptoms until I looked at Dr. Afrin's free chapter on MCAS.

Maybe fibromyalgia, but you didn't mention muscle pain, so it probably doesn't apply.

Obviously depression can cause somatic symptoms, so that's worth checking, but I think people jump to that conclusion too soon.

Note that a lot of these conditions don't really have tests, so it's really tricky to get a diagnosis. It takes finding a doctor who's willing to recognize them and give a diagnosis.