It sounds like you would want to disable each product's own thank-you email and keep only your Common Thank-you Email.
In the settings of each product, just check Enable Templated Email and leave the Email Message field above that blank (we can't generate an email from a template if no template is provided :^).
Then in your Seller Admin > Account Preferences > Common Thank-you Email, just make sure you include the code [%thankyou_link%] somewhere in your message template (the default template we provide there will already have this, so you might just leave it as-is). On that same screen, leave the Common Thank-you Page URL setting blank.
Individual products can either issue a file download or redirect buyers to a product-specific thank-you page URL (the latter is typically used to grant access to the "secret" URL of some premium or pay-per-view content, or of the entry page to a membership system), not both, because redirecting would bypass the thank-you page we generate with each buyer's unique download link.
You can add custom HTML and template codes to your E-junkie-generated thank-you pages, both on a product-specific basis and also in common for every order, as explained on this help page: