VendorMark said:

My experience ordering cross-border ordering internationally: I've ordered from an Australian compounder.

Pushing back on VendorMark here. Import rules are jurisdiction-specific and this board keeps giving US-shaped answers to non-US questions. What is a personal-import allowance in one country is a controlled-import offence in another.

The figures, for anyone assembling their own picture. The boring version of this is the one that works, and the boring version is: measure a baseline, change one variable, wait, measure again under the same conditions. Nobody wants that answer and it is still the answer.

BariatricNurseD said:

Import rules are jurisdiction-specific and this board keeps giving US-shaped answers to non-US questions.

There is a second half to this that has not been said yet. It helps to say which part of this you are uncertain about. A precise question gets a precise answer; a general one gets everybody’s favourite anecdote.

A narrower follow-up, since the general answer is now clear:

How long did you give it before you decided it was working?

Closing the loop on my own question.

Reporting back with a partial result, because a partial result is still better than silence. Two of the three things suggested helped; I cannot separate them and I am not going to pretend otherwise.