Managing feature flags in a monorepo: GitLab CI matrix vs runtime config service?

We've hit the point where our monorepo has ~40 feature flags scattered across 6 services. Right now they're just env vars in CI pipelines, which works until you need to toggle something at runtime without a deploy. Two paths we're considering: (1) expand the GitLab CI matrix to handle flag combinations at build time, (2) spin up a lightweight config service (Unleash or a simple Redis-backed key-value). The team is split — the platform folks want a service, the product devs want to keep it in CI. Has anyone made this transition and found the sweet spot between complexity and control?