Is this viable in 2021 without huge resources? Can you go ipv6? What's the startup cost of an AS like? I assume there's the technical cost, but what about actually getting an AS assigned (time/money?), etc?
Depends on what huge resources counts as, and where you're located.
Last I checked (which was a while ago), APNIC has enough IPs to give new applicants two /24s with justification which will get you started without spending a lot of time/money in the IP resale market. If you're in ARIN or the RIPE regions I think you need to go to the market (but also need to justify, so you're looking for a single /24 to start). I don't know of it's accurate, but blog posts and auction sites seem to show about $15,000 for a /24, which is kind of a lot, but doable. You'll also need to arrange some hosting somewhere that you can credibly BGP with at least two networks to justify having an AS... There are some providers that will tunnel you to an internet exchange point and services like Vultr that might help you get started without a lot of investment.
And you'll need to learn BGP on the job. But I think you can do a lot with a simple unix box, you may not need a giant cisco router these days.
You can do it small yes. ARIN et all will gladly allocate you ipv6, but finding a transit provider with full support is a bit more tricky. The ARIN fees to start up are a few hundred.
