40 ml is likely going to reduce your flavor.

You want a ratio of 50/50 or 60/40 with that device, and probably want to go with a higher concentration using nic salts.

I would maybe try and find a flavor you like (that would mix with the one you already have) in a juice that's 50/50 and mix them 50:50. Then you would have 70/30 which would work fine.

Adding plain pg will thin your juice but also make it bland.

You would be diluting your product by 40%, so if you want it to taste the same and have the same nic strength you would need to add 40% more flavor and nic than what the original recipe called for. If you dont, it will probably taste the same, but weaker, and you'll have somewhere around 2.4 mg/ml of nicotine as opposed to the 6 mg/ml.

You will get reduced flavor, you can warm you e-juice to about 45/50 °Celsius (sorry I do not know freedom units of measurement) and leave it at that temperature for about 3 hours it will make your juice thinner, please keep in mind that if you do not have nic salts this procedure will oxidize your nicotine and it may have a heavier throat hit as well will darken your juice... I reccomend a souis vide machine or a ultrassonic cleaner that can regulate temperature...

I have mod/rta combo that I run when I'm at work and it's not overly bulky. Pretty small actually. Augvape Intake on top of a Geekvape Aegis mini. You can get some pretty sleek/small devices that can handle 90/10.