What is the etiquette behind "thanking" partner for dummy?
As you say, whatever you do, be consistent.  Good contract, bad contract, plays itself, requires incredibly careful play, whatever.  If you do not do that, the opponents know.
Yes, sometimes that leads to some fun, after say 3C-X-p-p; p and partner dropping a 4=4=5=0 four count and hearing &quot;thank you partner, nice hand&quot; from declarer.  But just because it's obvious to the opponents doesn't mean that it isn't more obvious if declarer's response is &quot;well, that's not great&quot;.
Thanking dummy when it comes down is traditional, and courteous.  It is extraneous communication, not illegal, but not required either.  Many partnerships have given up on it, and just pause 10-15 seconds or so and call for the card.  If that fits with you and your partner, do that.  If partner feels that that is discourteous, find something else that works.
But do it consistently.  Any tell you give accidentally the opponents can use; a &quot;purposeful tell&quot; to mislead the opponents is illegal.  So you can only lose by not being consistent.
From a regulation POV, you have it exactly. Law 73D1 in the Duplicate Laws (my emphasis):

It is desirable, though not always required, for players to maintain steady tempo and unvarying manner. However, players should be particularly careful when variations may work to the benefit of their side. Otherwise, unintentionally to vary the tempo or manner in which a call or play is made is not an infraction. Inferences from such variations are authorized only to the opponents, who may act upon the information at their own risk.