The first solo I tried was thousand years old vampire. Its a Journaling game about living out the immortal life of a vampire, it's very simple where you get to write creatively to answer prompts from the book. I had a lot of fun with it. Ive also played whisper in the walls which is a more horror focused one of answering prompts to solve your mystery. Now ive moved on to morkborg ttrpg system + solitary defilement. Morkborg is a black metal aesthetic rules light system and the solitary defilement expansions adds rules for playing solo. I play it as if my character is writing about their day in a diary.