I am a 'newcomer' here. Just started a week ago with PTE AV Studio Pro. (Very nice program!) Some month ago I scanned all my slides (about 7000) too. I did that with Silverfast 8 and a Epson Perfection V750Pro. In the Silverfast program you are able to set the borders in a percentage of the total scanned size (and of course much more settings). And last but not least: Silverfast has developed a complete new job program (I was finished at that moment ...). The site of Silverfast: https://www.silverfast.com/ After finalizing the color and the final framing of the pictures with Capture One I am busy to put them in shows with PTE.
Greetings and success!