I use for my clients Amazon SES in order to send emails. So for each client I validate their domain. The problem is that the TXT record to validate the domain is _amazonses I would like to know if there’s is way to customize it or to change the way of validation so my client don’t see I am using Amazon SES
This can't be changed. Also, it's just one of the things that customers will need to change in DNS.
You will need to have your customers update their SPF records or you will have significant deliverability issues. DMARC (mentioned at the same link) will pose additional challenges, in some cases, and providing the wrong advice to your customers on their settings can break their ability to send email via existing channels.
Sending mail on behalf of your customers using their domains is distinctly non-trival and best avoided -- SES or not. Regardless of the vendor, you won't be able to mask it.