Recurring large amount of dark rectal bleeding inc. large blood clots. colonoscopy showed only internal hemorrhoid. ok to assume blood only due to it?

Unlikely : The bleeding is probably from diverticulosis or something known as an arterio-venous malformation (AVM). Since these can be anywhere in the colon, if surgery is ever needed to remove that section, it is important to find the exact location with a variety of imaging tests, usually only helpful during a bleeding episode. Hope this helps!