Just Google “multitrack cassette transfer”.
Looks like maybe $30-80 per cassette.
It’s easy to transfer to .wav files. It’s done using a multichannel analog to digital converter/DAW interface. It just needs to have 8 simultaneous inputs, and then one has to have a DAW that records 8 or more tracks at once. Which pretty much is anything nowadays. And then of course you have to have a working Tascam format 8-track cassette machine with dbx to reproduce the tapes.
I have a refurbished 238, MOTU 8M and a suitable DAW.
How many tapes and of what length do you have?
Using a 424 will not work. The track spacing is totally off…the 4-track will only reproduce tracks 1, 2, 7 & 8, tracks 1 & 8 don’t land all the way on the heads for tracks 1 & 4 of the 4-track so there will be some loss of fidelity of those tracks…not sure if the dbx will decode properly, and because of the staggered head design of the Tascam 8-track format tracks 1 & 2 will not be in sync with tracks 1 & 8.