Well it's not packet. There's your problem. You should select normal USB and the recommended dial frequency as conveniently listed in the program. You need no more than 2 kHz bandwidth as the JT65 "band" starts 2 kHz higher than FT8.
The computer/tablet/smart phone will produce the tones with a base frequency of whatever you select. in the range of say 300 Hz to 1975 Hz. It's probably best not to go below 300 Hz as the IF band-pass of the other guy's Rx will likely reduce the signal (but not the S/N) below that.
You need a keying signal, most of us use an audio derived switching signal to operate a FET or BT. I suppose you could use the transceiver's VOX but I haven't tried that.
The main difference between different A-Ds is in the number of bits. The more bits the greater the dynamic range and the better the sensitivity. There is an optimum sampling rate too. This are explained in the Guides.
FT8 is intended to be quick(relatively) and easy and very sensitive. That is no effort to learn a code but able to work stuff you can't hear. Easy doesn't equate with bad. The majority of DX stations have voted in favor of it. It will gain acceptance in SOTA too.
I think the fish are safe in most barrels, it sounds like a no brainer, until you try it. I shall say no more lest I get into trouble.
Have fun and get your fish at a shop.