Cancellation of construction contract is not confirmed

You do not have to confirm anything.

You are of course right. But if I ask for confirmation, then I also expect a reply.

Otherwise, a written declaration by registered mail will follow in due time.

A registered letter (even with return receipt) only confirms that someone has received an envelope. Therefore, one should always have a good witness who can confirm what was sent.

A return receipt is even the worse option, as there is the possibility that the mail carrier does not find anyone and only leaves a pickup notice. If no one picks up the letter, it is not considered delivered.

A better option is a registered letter with delivery confirmation, as it is considered delivered upon insertion.

Fax with protocol was also good.

The safest is by messenger, and in such a way that the messenger knows the contents of the envelope and can confirm it.

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You always do something like this (additionally) with a registered letter (with return receipt/hand-delivered). With a revocation by email, there is still no uniform direction of the courts (for proof that the revocation was received).
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But a registered letter says nothing about the content.

Very important mail I have delivered by a bailiff. That is the only "foolproof" way to provide proof and quite affordable.

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The safest way is by messenger and in such a way that the messenger knows the content of the envelope and can confirm it.

see above

Or with a registered letter with witness(es).

A registered letter says nothing about the content.

However, you will not lose in court with a revocation that was already made by email before and sent as a supplement – the claim that the recipient did not receive the revocation is almost impossible to succeed.

P.S. I only send such things by registered mail with return receipt signed personally.

It would still need to be distinguished whether the revocation must be made in text or written form. If the latter, the email is already irrelevant.

However, I agree with the previous speakers. A confirmation is not required.
If I have read correctly, you have already signed with the second company, meaning you have already accepted the risk of exceeding the budget if damages were to be claimed.
So only the feeling remains that something might still come up.

It would still need to be clarified whether the revocation must be made in text or written form.

Text = email is sufficient