The solution to my problem ended up being fairly simple, but took long enough to solve that I feel it's worth answering.
Below is the amended, working code:
var feats = vectorSource.getFeatures();
// Takes in multiple features and combines them into one
var geom = joinGeoms(feats);
geom.transform('EPSG:3857', 'EPSG:2883'); // Transform into CRS specific ...
It's now part of the buffer tool. You can now enter a buffer value in the distance text box or you could click on the button on the right of the distance text box to set a variable distance buffer( by selecting a numerical field or with an expression)