with your current setup you can likely run 7,8 or even 9spd wheels in friction mode. you may be able to simply switch your shifters to shimano 7 or 8spd DT shifters and it is possible your derailleur will work with the shifters and wheel.
I have an older Bianchi I am doing the oppisite I have suntour shifters and freewheel and shimano derailleur.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk