Nicotine salts (they're a less harsh form of nicotine than freebase and with less throat hit). I'm not quite clear on the directions here, to the OP, but if you use nic salts you CAN often go higher, although ideally you don't want to wind op overdosing and Yes, your brain adjusts to nicotine in your vape just like it did when you were smoking. Any smoker's "buzz" would more likely to be from carbon monoxide than anything else, not the nic.
It is true that your tolerance can rise (and fall) rather fast, most vapers (surely not all I guess) are going for a nice even stream of nicotine that is neither too much, nor too little. If you do want that "buzz" you'd have to raise or lower your juice, but I could also see that getting somewhat tedious as well, and hard to manage.
You can MIX juices in your device, but depending on strength, whether the flavor matches the flavor already there and your PG/VG level you may wind up with something harsh, that burns you coil, that is not the correct viscosity for your device, that tastes horrible, that is too high or too low for you making it hard to vape and etc.
Best of luck,
Anna