Well, they call it a blending of the minds in that TNG episode (“The Host”), but once Riker takes on the symbiote, he doesn’t retain much of himself at all. He’s all Odan (the Trill character). Riker is only able to host the symbiote briefly before becoming very ill. Once the new host arrives, they take on Odan’s personality completely. ¯_(ツ)_/¯
The Trill were obviously in a prototype stage for that episode, as they appeared without spots, and with forehead ridges instead.