Most tools have a lot of overhead. As a solo dev, your advantage is speed, but a little friction and grist is needed sometimes.
It takes only an hour or so to restructure it for more than one person, so cross the bridge when you get there.
For to do lists I use Sublime+PlainTasks plugin. It's fast, sticks in your head. If you want something cloud, Workflowy might be good too.
Without a product manager or scrum master it's also easy to get into tunnel vision and stray. I sometimes enjoy having an extra person to bounce ideas off and tell me to keep on track. If it's a startup, get a co-founder, give them 10%-50% just to tell you listen to you. It seems a little unfair, but it's worked for me.
Another trick would be a design doc to remind you of design decisions. I'm happiest with this format: https://random-character-generator.com/whatisthis