@infinitejest im a full time sound engineer and ghost producer. I currently am using a 2019 MBP i9, blah blah 15 inch spec'd out. My projects currently hover anywhere between 25-60 channels of audio and AU's. My sessions are stacked to the ****. My advice is this; Upgrade your 2015 to what you need, not what you want. If you can find a refurb 2015 get it. The peripherals are a god send still for what you will need. If you can get a well priced 2019 15 inch then do so. Mine has not skipped a beat.
Just keep in mind for audio work, especially in Logic Pro which is optimised for Apple products, it can literally sail along on single core or dual core machine as
@Howard2k just explained. Don't buy into the hype.
Just my 2 cents