I've interviewed all but one member of my team (the one I didn't interviewed me)...
As an employer, putting this like "Gotta" in an email to the CTO, during the hiring process, is an immediate no. After you've got the job, be as friendly as you like.
From the other side of the table, as the applicant - you went way too far.
If the spec says 3-5 hours work and it takes more than a day, that's a red flag that the company doesn't understand complexity. Should you accept a position, you'll never have sufficient time to complete requirements, and they'll likely expect free overtime to meet unreasonable demands.
The fact that the CTO said "no problem with the approach" yet the senior(s) had a lot of negative criticism also tells me that the seniors don't share the CTO vision, and that's another big red flag.
Overall, I think you dodged a bullet.
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