In the beginning I thought I would need to get a tobacco flavor juice in order to give myself the best chance at quitting the smoke. You know, emulate cig as much as possible. Wasn't the case.
Getting used to inhaling vapor (70/30) instead of smoke was enough for the "hit". That first week I started at low wattage and went up a little at a time (as I got used to vapor) for more hit, smoking less as I went. 18nic was a little much, like oversmoking, and went down to 15.
I would recommend a MTL set up for someone like your father over a Direct to lung inhale.
I know the nautilus series of tanks are well loved by MTL vapers,
^This^
Cigs and cigars aren't DTL. He's tried your 2 diff setups and didn't like it because it was DTL. But at least he tried them, so that tells me he can be converted. Not very often does someone go from smoke to DTL vapor. MTL is almost always in between there.
Both, your AIO and V8 stick are DTL, and love it or leave it, because you can't control the output. Not good for beginner because they need to start extra low on wattage so they don't cough, and gradually go up as they get used to it.
Get him a good MTL tank and a variable wattage mod. It may not be a kit since usually atty/mod separately allows better choices worth that extra $.
A worthy kit would be Aspire Zelos Kit. A built in battery similar to a single 18650 without needing to buy or recharge external batteries, and a highly successful MTL tank. Triton Mini coils are cross compatible and I'd get that Clapton 1.8 then he'd have two 1.8 coils and the .7(I think) that comes with the kit. Use the 1.8 and start at 6 or 7 watts so he doesn't cough, and bump up a little as needed.
The original Nautilus is 5ml if you think more juice capacity is better.
Another option imo would be Typhon built in battery mod for 100w of future proofness if he continues to DTL, and get the tank separately.
For juice flavors, I'd try something like his favorite desert or gum.
MTL + variable wattage mod.