If it will help with your back and overall comfort, do it! It should be completely safe for your Esteve without compromising structural integrity at all. I've installed strap buttons on several classicals without any issue. If you have a drill and might be willing to do it yourself, I'd definitely go in this direction as its a very easy/quick job, will save you money, and you don't need to leave your guitar in someone elses hands. Let me know if you'd like some instruction on how to do it.
As for button placement... I definitely recommend putting a button on the bottom of the lower bout and at the side of the heel as shown here: https://www.cordobaguitars.com/live-pla ... tar-strap/
- since this is a MUCH more comfortable way to use the guitar with a strap rather than tying at the head stock or using a sling. Doing it this way allows you to practically have the guitar in standard classical position while standing.
The only drawback of strap buttons however is it is not exactly traditional in the classical guitar community, so it could be more difficult to sell the guitar in the future, but not impossible. If you plan to sell your guitar anytime soon, then I'd hold off.
On another note, have you ever tried using the guitar with a guitar support rather than a foot stool? This can also be a comfortable solution.
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