Optimizing table for timeseries Postgres data table
If you were seriously considering switching database systems, I could tell you Microsoft SQL Server already has an out of the box feature for sparse columns that fits your use case we'll. But my recommendation would be to not look to change database systems only just to optimize data storage. PostgreSQL is a very capable database system itself.
To answer your question, NULL values only take up 1 bit of data regardless of how many columns there are, and therefore is very lightweight so it reflects your sparseness accurately.