I really want to stress that solo rpg is one of the cheapest hobbies there is. So many rpg systems and solo systems are free or PWYW. Once you wrap your head around it, you just need a way to write things down, and some dice--and plenty of systems use six sided dice, so dice from any board game will work. Even D20 games can be hacked--roll 3 six sided die. You won't get results of 19-20 or 1-2, but that just makes your results a little less swingy.
Most of all, it's about emergent storytelling.
I've settled into using fountain pen, pencil and physical dice for most things, and using my tablet for rules reference and random tables. Being as analogue as possible is helping me enjoy it, I think. It's become precious quiet just-for-me time No one will see the results, no one will judge how I do or how I journal, it's me and the dice and my imagination.
My two current campaigns are:
Classic Traveller (facsimiles of the 1981 editions of the basic books are free, but I bought a huge bundle of supplements): I think I have spent roughly 8 hours and my main character has done two (2) things so far, bought some spice and found a patron.
Constructing elaborate stories to support character generation results and rolling and developing a subsector has been wonderful reflective creative practice. I find myself thinking a lot about it when not playing. While Mongoose Traveller felt more of a complete rpg in the modern way, Classic Traveller feels like a box of tools to create my own stories with. It's going to be amazing for solo, I think. It's already been amazing if I never play it again.
On the fantasy style of things, I've been using Microlite20 (PWYW, I chucked a couple of dollars but the suggested price is free) which is a super simplified system for D&D 3.5 compatability (other versions exist if you want to play other editions or systems). I generated a random dungeon on Donjon (free, and it also has a free copy of the Purest Essence version of the rules). I'm using Plot Unfolding Machine and Scene Unfolding Machine (paid) as oracles, but a free Oracle like One Page Solo Machine would work just fine.
I've found the combination of random dungeon and straightforward rules really effective. My brain, given necessarily limited prompts, is completely happy to weave them into a coherent story and add meaning to the dungeon.