The slow part of a Mongo migration is restructuring the data, not moving it. Once the schema is set, loading seven years of hourly data is a script and a progress bar, not another multi-month project.
The core of this is free on self-hosted open-source TimescaleDB: hypertables, compression, continuous aggregates, retention. Only tiered storage is Tiger Cloud only, and it’s marked below.
If you just want to prove the migration is fast, spin up a Tiger Cloud free trial, load a slice, and time it. Same SQL runs self-hosted later if you’d rather own the box.