You can definitely serve way more than a few GB per month through Cloudflare on the free plan. I serve tens of terabytes a month for free. If OP needs to serve hundreds of terabytes per month they may get an email asking to upgrade, but the backblaze/Cloudflare setup would probably still be the cheapest. BunnyCDN is great too.

OTOH I've been told (by CloudFlare support, in contact with their engineers) that for their "for hosting game levels and other content" use case[1], any of their ordinary plans should be fine.

I'm not... super confident in that answer, because despite that being a use case they promote on the site the terms seem a bit murkier, and the page on that use-case doesn't say much about which plan(s) they expect you to use (I'd have expected an "enterprise" plan for serving hundreds of TB of transfer of game-assets per month, but they said no, any normal plan's fine, which... I was up front with them about what our usage would look like, and they held that line, but that seems too good to be true).

I haven't tested these claims yet.

[1] https://www.cloudflare.com/gaming/