I wouldn't accept this switch, you are correct in that it is dose shrinkflation.
Lenzetto is known to be quite weak, in fact weaker than it was initially thought to be compared to other products.
Gel also makes blood tests unreliable due to unstable plasma levels, if this is something that is important to you.
If the mirena IUS is the only thing that has changed recently and your mood has deteriorated, this may well be the culprit.
Are you on testosterone? If not, my reply to being told my testosterone was fine would be "great, the only value of a baseline test is to exclude a high level, which it has just done, and I would like a trial of treatment now please based on clinical symptoms".
Many women find testosterone beneficial for mood and cognition, however you would have to say it is for libido on the NHS.