There is a screamingly obvious answer here and I don't understand why they aren't taking it -- raise prices, make the free tier less generous. Either slow down the growth or up the revenue pipe so you can throw more compute at it.
Raising prices risks churn, reducing free tier risks lowering customer acquisition rate.
You can free tier but still sensible rate limits and QoS policy. You can have separate region or cluster for your paying customers. Finally you can spend time re-architecting GitHub Actions or just aquehire one of many competitors that do better co/cd.
Not remotely compared to github, and you can run GitLab OSS internally anyway.