There is already a blacksmith sh if you don't want to selfhost your runners. Or codeberg as a full solution if I am not mistaken.
I have it for all my stuff (dozens of repos and hundreds of PRs/builds per week) and it's only once in the last year that I've exceeded the free resource levels. I don't know if it's just extra capacity or they're burning VC dollars in the hopes of inspiring posts like this, but either way I feel like I'm taking advantage of them at this point.
If you're looking for a publicly-accessible Forge SaaS for your private repos and are willing to pay for it, you are almost certainly a company developing proprietary software.