Mail "Send to" vs "Send to all"

I understood your question perfectly.

Reply to All puts the original sender in the To: and all other recipients in Cc: - that is just how it works.

Everyone will receive your message, so this does not really have a big impact.

However, you seem to be keen on putting everyone in the To: field, instead.

I don't see a way to do that automatically, but you can easily drag a rectangle over the addresses in Cc: to select them all, and then click and drag to move them to To:

When sending an answer, choose the second arrow instead of the first one:

I receive dozens, sometimes hundreds of e-mails a day, and I hardly ever notice whether I am in the To: or Cc: of those messages. I don't care. But I can imagine that some people may treat them differently.

FWIW, I think that the way Mail handles it makes total sense.

After all, if A sends a message to you (B), and also to C and D, and you choose "Reply All", you are actually replying to A - whose sent the message - and including C and D in the conversation - hence they belong in the Cc:

Since I don't know of a way to make Mail behave differently, at least you know you can easily drag the addresses from Cc to To.