Get a personal domain and start all your side projects as a subdomain of that top-level domain.
Do not buy a custom domain for every little idea you have. Save your money, give your project the longevity it deserves, and show off who made the dang thing implicitly.
Your personal domain is the one you’ll own the longest, so you can easily upgrade a side project subdomain to a top-level domain if/when it becomes reasonable enough to justify it for X, Y, or Z reasons (simply setup up a long-standing redirect from the subdomain to the top-level domain).
However, you can’t easily “downgrade” from a top-level domain to a subdomain because eventually you’ll stop paying for the top-level domain and lose the redirects from any residual traffic.
I also think it’s cooler to see small projects / microsites / whatever you want to call them at subdomains. Couldn’t tell ya why, I just like the vibe. And for the record, it’s not like you even have to share the same host. You can point the subdomain wherever.
Money where mouth is:
The smashburgers one is Cloudflare Pages, the breakfast burritos one is Netlify. I had to go look it up because I couldn’t even remember. Once you set up these site builder tools that deploy on git commits, they tend to just keep working.