And more to the point: I will fully trust the online tax software if it's free (libre), secure against eavesdroppers, and operated by the tax-collecting government agency itself.
Eh? How is that supposed to work?
Either the online website is operated by the government agency, then you have no way to know that the version of the code they run is the version they make available for download and scrutiny.
Or it's libre, you download and scrutinise it, and run it yourself, in which case it's not operated by the government agency (and isn't particularly online any more).
Also, why would you want libre tax software if you only want a single source to run it?
Isn't the entire point of libre software that it can be modified... by whom, if you won't trust any other service suppliers?
If you want a single, trusted supplier and are only concerned about being allowed to scrutinise the source code, then surely proprietary but gratis and source-available would be sufficient? (Though that still wouldn't solve the problem of proving that the code you see and the code you (implicitly) run bear any relationship one to the other.)