I’m afraid this is right: it’s hopelessly unrealistic (to put it gently) to suppose that a U.K./EU customs union could involve any form of joint decision-making.
Note too that while a single market arrangement could give the U.K. various rights to refuse, or adapt, new single market measures (the EEA has various mechanisms for this) it’s hard to see how that could work for a customs union.
A customs union requires full consistency of tariffs, rules of origin, and classification: it’s a common wall, and if treatment of goods is more favourable than it should be at one part of the wall, that’s where goods will flood in.
So it’s hard to see how any U.K./EU customs union would materially differ from Arts 3 and 4 of Annex 2 to the backstop protocol. 
Note for example Art 3(1): “the U.K. shall align ...”; the “may not”s in Art 3(2) and (3); “shall be *informed* of any decision to amend” tariffs (Art 3(4)); the right (only) to be “consulted” on trade defence tariffs (Art 4(3)).
There is little basis for believing that any permanent customs union with the EU capable of being negotiated by any U.K. government would look much different.
