veganbikes

It could be a lot of factors involved in a shop not wanting to work on the project. Space is a huge factor and being able to mount it to a stand easily and safely is another. Most shops do not have the ability to deal with recumbents much less a tandem recumbent and maybe they would work on it if they could but it is a lot more work. Plus adding odd mounts for things that do not exist on the bike and are safety related is not something any shop is going to touch unless they are frame builders and are welding something on. A recumbent shop probably has a stand for a recumbent and could work on it but then again they probably don't want liability in installing some jenky add on disc brake kit (and I am not talking specifics I am just talking in general) I know I wouldn't have any of my techs install something like that no matter what money was involved as the risk is too high. Just because someone does sell something doesn't mean you should buy it.

I would post pictures and info on the bike so we know what we are dealing with and can make recommendations for brakes that are going to be safer.

In terms of IGHs they are great but Rohloff might be your only bet and even that is not something I would really want on a tandem personally (though I do love Rohloff) I think the added stress for touring would be closer to the limits on that hub. I would stick with a good high quality derailleur system. Shimano XT would be my top choice but if you wanted road components 105 or Ultegra would be a fine choice or GRX in the 11 or 12 speed spec.

Gates Carbon Drives are excellent but do require either a gearbox (Pinion) or an IGH or to be run as a single speed. For tandems they are typically used for timing but unsure if that would work here but if so I would recommend it for timing it does cut out weight and maintenance and costs less over the long term generally especially on a higher used item.