While I don’t like the side effects, so many drugs cause similar issues (memory issues, headache, brain function), like statins, yet people keep taking those!
I found tamoxifen side effects to be dose dependent. I had headaches and brain fog at 20 mg. Very tolerable level at 5 mg.
There are benefits to tamoxifen, as well. It provides some protection to heart & bones. It does not damage collagen. For me, it has reversed GSM (vaginal atrophy and dryness). My skin looks better on tamoxifen - most likely because of the estrogen like behavior. If I were around 55 yr old and postmenopausal I’d be eager to try tamoxifen because of research showing how estrogen is beneficial if started shortly after menopause- and tamoxifen is an estrogen agonist in most areas of the body but blocks receptors in the breasts. But I started tamoxifen at 19 years post menopause- so a bit unknown.
It can be a problem for those who are higher risks for thrombosis and endometrial cancer, though.

I’m still working through the headache issue, I’ve had mild intermittent headaches on 5 mg. I started using a nasal antihistamine spray (vasoconstrictor) in March and do not have headaches. There was some indication that I was having sinus pressure (not in my cheek/eye area, and no nasal congestion) - so maybe the recent onset of headaches was only pressure. But tamoxifen is a vasodilator and vasodilation can cause headaches - my theory is the antihistamine is modulating the tamoxifen headache. But that’s probably a leap - and difficult to prove even if it were true.