first off the split ends gotta go. they’re not doing you any favors: they’ll just continue to split up the hair shaft and you’re hair will look thicker and fuller without them.
are you looking to thicken the hair strands themselves? dry shampoo or a thickening cream will help your hair -look- fuller by coating the hair strands. biotin hair products are out there but I don’t know how well they work.
if you’re trying to regrow hair, minoxidil is really your only option. you place it on your scalp but it’s fatally toxic to cats so if you have cats, you can’t use it.