In bridge, Ethics are defined by the Laws (and Regulations, created as allowed by the Laws) in what used to be the Proprieties section (Laws 72-75). The relevant part of those laws to coffeehousing (a slang term, meaning "varying tempo to pass a potentially misleading message to the opponents") are 73D and E, quoted in full here:
D Variations in Tempo or Manner
- 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.
- A player may not attempt to mislead an opponent by means of a question, remark or gesture; by the haste or hesitancy of a call or play (as in hesitating before playing a singleton); by the manner in which a call or play is made; or by any purposeful deviation from correct procedure (see also Law 73E2).
- A player may appropriately attempt to deceive an opponent through a call or play (so long as the deception is not emphasized by unwonted haste or hesitancy, nor protected by concealed partnership understanding or experience).
- If the Director determines that an innocent player has drawn a false inference from a question, remark, manner, tempo or the like, of an opponent who has no demonstrable bridge reason for the action, and who could have been aware, at the time of the action, that it could work to their benefit, the Director shall award an adjusted score.
"Demonstrable bridge reason" is given pretty big leeway, especially when dealing with "I don't have a problem yet" situations. Partly because thinking on the first (or third-last) pitch of an expected suit run might just make it clear that, yes, you did have a problem on this trick - see how many "oops, you misplayed at trick 1, the hand is now hopeless" articles there are - but also because you do have a problem - "what pitches am I planning on making, what order of those pitches will be of best assistance to partner/least revealing to declarer/keeps my options open in case of declarer stopping running or partner making an unexpected discard?".
And if you play a special first discard style (like lavinthal or odd/even) then obviously you should be thinking not only which cards are going to be pitched, and which one gives the message you want, but whether you can afford the "right" first discard. So you're even more expected to be "thinking about the rest of the hand" at this point.
Yes, it "masks" which are "easy" discards and which are "uncomfortable" discards to declarer. See "the easy fifth" (which I first read about in Sabine Auken's I Love This Game) for instance. But "thinking at the start" is not an "unwonted hesitation" - everybody knows that that's where you should be thinking, and the information it passes is "I need to find 5 discards".
Of course, you want to be careful when it's your last follow rather than the first discard. Of course you don't want to be tanking on every trick. But it is not required to telegraph your problems to declarer, and it is a valid bridge reason to be "thinking about all my discards" now. That helps in "maintaining steady tempo and unvarying manner" - especially if you make a habit of always taking a little extra time before the first discard.
Postscript: @ruds mentions the face-to-face pattern of "refusing to quit the trick" to imply "I need to think about the hand now, but there was nothing to think about on that play". It's a good pattern. Unfortunately, there's nothing in the Laws that stops the winner of the trick from leading before the previous trick was quitted (yes, I was shocked when I found that out too!), just that your right to inspect the trick ceases when you turn your card over (66A). And there have been multiple times when I said "I'd like to see the trick again" after the next lead (but before partner played); there have been multiple times when I was still thinking about trick 2, and they played trick 3 (to which I "followed" with the same card I played to 2) and then led to trick 4. So, good pattern, but "It Doesn't Always Work".