I know how addictive making envelopes can be! Once I get started I get in a robotic state. The cutting, folding, gluing etc are a little monotonous but in a therapeutic way.
Just wondered if you'd thought of trying envelope glue as opposed to double sided tape?
For really old books I do use double sided tape, but when i'm making envies from fresh, store-bought/magazine freebie papers I use an envelope glue called "lick n stick". it's quite time saving.