first off the split ends gotta go. theyโre not doing you any favors: theyโll just continue to split up the hair shaft and youโre 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 donโt know how well they work.
if youโre trying to regrow hair, minoxidil is really your only option. you place it on your scalp but itโs fatally toxic to cats so if you have cats, you canโt use it.