Creating the Second Guess Strategy That Will Actually Narrow the Options Down

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Every strategy guide for Wordle Unlimited always starts off with the explanation of the most efficient first guess and then stops there. It is completely the wrong way. First guess problem is thoroughly solved, and the gap between the first and the tenth guesses is insignificant. The game is usually won or lost at the second guess, and people hardly pay any attention to it.

The Mistake Almost Everybody Makes
You have just made a guess of CRANE. Green A appears on the position three, and all the other letters are grey.
It sounds reasonable to make the second guess of this A, for example, PLAID, BLAST or SHAKY. It sounds logical, as you use the gained knowledge.
However, it is the wrong move, and here is why. You already know the position of the A. Using the word with A in the third position gives you no additional knowledge about the letter and takes four positions for new letters instead of five. You just wasted one fifth of your guess checking something you already knew.

The Rule: Use Your Guesses to Reduce Uncertainty, Not Certainty
The value of each guess lies completely in uncertainty reduction it provides. All the letters you have found contain zero uncertainty, and their usage is mostly wasteful.
Having played CRANE, you know the location of C, R, A, N and E. You know nothing about S, T, L, O, I, D, P, U, M, H, B, G, F, K, Y and W. Your second guess should consist of the latter list.
For example, MOIST, PLUMB, DIRTY, SHIFT, TOPIC, LUMPY, DEPTH could be the good guesses after CRANE. All of them have the same feature – they overlap little or not at all with typical openers.

Exceptions to the Rule
It is not an absolute, and these exceptions are crucial.
Apply the rule when you have three or more green letters. When you get S_ARE with four letters already known, the exploration is useless. You do not need to guess anymore, just choose between the few possible candidates.
Forget the rule in hard mode when you have to – you cannot help it and you have to reuse any letter which was revealed.
Forget it in time-limited game modes when fifty percent chance to win now is better than one hundred percent win on second guess.
And forget it when the pool of possible letters becomes too small. Four orange letters after the opener mean that you are very close, and the further exploration will only delay the solution.

Designing Your Pair of Guesses in Advance
The perfect way to play is to design both guesses and use words with no common letters.

First guess        Second guess  Letters covered

SALET

CRONY

S, A, L, E, T, C, R, O, N, Y

CRANE

MOIST

C, R, A, N, E, M, O, I, S, T

SLATE

CHIRP

S, L, A, T, E, C, H, I, R, P

CRATE

POUND

C, R, A, T, E, P, O, U, N, D

Ten unique letters with two guesses, covering all the frequent vowels and majority of valuable consonants. After the second guess you will reduce the candidates list to just a few options, and the third and fourth guesses will become a matter of convergence, not exploration.
The tradeoff is quite obvious: you lose your chances to solve the game on the second guess, since you ignore the information you have just gained. This strategy is good for the players trying to minimize the average number of guesses. People preferring to win in two guesses will not like it.

Correctly Reading Orange Tiles
The second guess is also the stage when the positional information is ignored most frequently.
Orange tile contains two types of information, and one of them is often ignored. The letter is in the word, but it is not in this position. If your opener shows you orange R on position two, your second guess should have this letter somewhere else. Having it on the position two means zero information gained.
The opposite is also true for the grey letters. Grey letter is not in the word, and its reuse is wasteful utilization of a slot. It happens more often than many think, mostly due to some word sounding right.

How to Improve This Skill
The second guess is hard to practice systematically, because the daily puzzle allows one opportunity per day, and the next day you forget what you were practicing.
Unlimited rounds in wordleunlimited.dev solves this problem. Helpful exercise is to play twenty games with conscious choice of the second guess with no overlaps, and then another twenty with building your guess using the letters you have received, and then compare where you stand after the third guess. Many will be surprised with the results.
Increasing word length to six or seven letters makes the lesson much more clear.