Have you tried ecommerce sites on mobile? With the amount of typing on a pokey little picture of a keyboard that hides most of the page I’m on, box ticking, large blobs of text to read etc I don’t think I’ve ever completed a purchase using mobile.
I may do an initial product search on mobile but go to PC (or tablet is not quite as bad a smartphone) to read more and order.
You should head for a responsive site but short term have some stripped down mobile-only content. The mobile-only can offer to open the full-fat desktop version but basically it’s to do the first steps of the marketing job: https://en.wikipedia.org/wiki/AIDA_(marketing) where the final “Action” is “call us” or “go to the full-fat website”.
“Responsive” is interpreted differently by different people. For example, some will display the entire “normal” content by taking a 3 column page and rendering it as a single column 3 times the height, better (IMHO) is when the central column contains the key content, right and left cols are non-essential supporting info and they are not rendered at all on mobile. OK that’s a simplistic example but essentially address the mobile audience in a way best suited to their chosen platform. That probably means simpler, less content.
“…emotionally attached to it …” Don’t be, a 5 year old site has earned it’s keep, you have to move on sometime and in 5 years a lot has changed, take the challenge and implement some exciting new stuff before a competitor beats you to it.