Terrantula + Terraform Cloud / HCP Terraform
Keep TFC. It hosts your runners, your state, and your run pipeline — Terrantula does none of those things and never will. Terrantula is the layer above: it decides which Terraform to change, where the next instance lands, and tracks the population over time.
How they compose. Terrantula Actions open pull requests against your VCS-connected repos, and TFC's own VCS integration picks the merge up and runs the apply — no new pipeline. For workspaces you drive by API, a TFC run trigger fires the run directly and tracks the outcome.
What Terrantula adds that TFC doesn't have: a per-tenant entity model, cell placement with capacity ceilings ("least-loaded cluster, max 50 tenants"), relationship cardinality, and lifecycle across hundreds of stamped workspaces. TFC's workspace list is the inventory; Terrantula is the fleet model over it.
One honest cost note: TFC bills per resource under management. Terrantula doesn't change that bill — but its fleet view is where you finally see what's driving it, per tenant rather than per state file.
Not for you if you have a handful of hand-curated workspaces and nothing stamped — TFC alone is the right amount of tooling.