The problem is not with the mailto: but with the format of the link. If you format your email link like this:
Then you will run into the issues you described. Users may just want to read or copy the email address but they can't (easily) because the only option is to click the link and leave it up to the browser/mail client to handle it. However, if ...
How about showing the suggestion as a clickable button/tag? This would invite the prefill while keeping the user in control (should they wish to enter something else). https://www.instagram.com/p/B1FbvXAg1Jp/
Allow two options only, and make the Smart Default by default selected
The goal is to speed up the process by using smart defaults, then make the smart default option by default chosen. Allow two options only: First is your Smart Default. Second is creating their own.
Remember to make sure that the smart default you are using is as good of an option as ...
So far these are the two options that I could think of, which seem to offer a good fuctionality at a click/tap distance and with no apparent harm:
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The suggested text can be light gray, so that it's very clear that it's a suggestion.
If there's already a help text at the bottom to clarify the added text is a suggestion, add a line to tell the user they can modify it.
Essentially, your users can either select one of your suggestions, or input their own custom name, so I'd present all of these in a single radio button list. This would present your automatic suggestions, but also communicate the option of customizing the embossing name, all while still limiting them to a single selection.