With the benefit of starting basically from scratch and not having to mess around with real-time analytics, it's pretty easy to ignore the MDS vendors. So far I've landed on BigQuery, AirByte, GitHub, BI Engine, Looker Studio, and Pandas 2.x or DuckDB for local stuff. I send as many things as possible straight to BQ, lock junior analysts out of gigantic tables, archive periodically to partitioned parquet files in cold storage, use mostly turnkey integrations, and ruthlessly prioritize custom ETL jobs. Putting GitHub in the mix isn't super ergonomic and we may be in the market for new tools once we cross the "big data" frontier, but that'll be a while from now. I'll probably never know or care what the MDS vendors think I'm missing.
gtm javascript loads, pulls down the config, injects tracker X javascript into the browser
new gtm:
gtm javascript loads, pulls down config, streams events to google servers to fan out to tracker X as configured
So blocking gtm.js off tagmanager.google.com / www.googletagmanager.com / the various other domains still blocks all gtm injected tags.
The tl;dr is they're become much closer to segment -- which does the data fanout internally to segment. But they should still be straightforward to block.