The words “simple” and “email template” don’t go together in the same sentence.
If you are sending out the emails yourself, then as posted, plain text is your best bet for having the least cross-client problems. If you know for certain that all of the recipients will be using known email client(s) you may have some luck coding for them.
But if you want good looking emails with the least amount of hassle, instead of looking for a template for you to use, you should look for an email service. Depending on how many you are sending you may be able to take advantage of a “free tier” some of them offer.