熟考 and 熟慮: Deliberation, replies, decisions, and wishes
Compare careful thought about proposals and an important contract. Treat overnight or several-day deliberation as example details and distinguish a reported reply or acceptance decision from a wish to reply after consideration.
A reply and a changed plan after consideration
熟考 includes careful consideration or deliberation. Its examples report an overnight consideration followed by a reply and a changed original plan after careful thought.
「提案を一晩熟考してから返事をした。」 (I considered the proposal overnight before replying.) The proposal was considered overnight and a reply was made. Overnight is the duration in this example, not a required duration for 熟考.
A decision to accept and a wish to reply
熟慮 also includes deliberation or careful consideration. The several-day proposal example reports deciding to accept, without establishing that a contract has been signed or fulfilled.
「重要な契約なので、熟慮してから返事をしたい。」 (This is an important contract, so I want to consider it carefully before replying.) This expresses a wish to consider an important contract before replying. It does not report that the consideration and reply have already been completed.
The stated duration does not define the word
The changed plan, decision to accept, and wish to reply belong to separate situations. Neither word imposes a fixed outcome such as changing a plan or accepting a proposal.
The meanings overlap in careful thought. Instead of turning overnight and several days into a word-selection rule, inspect the actual subject, duration, and stated reply, decision, or wish.
How does replying after considering a proposal overnight with 熟考 differ from wanting to reply after 熟慮 about an important contract?
Show the answer and why
The first reports consideration and an actual reply. The second expresses a wish to consider the contract before replying. Do not make overnight or several days compulsory durations or turn the wish into a completed response.