First, your hexoxhm v3 is not a mechanical, it is a quasi regulated with crude basic protections that uses a potentiometer to adjust voltage going to the delivery device.
There isn't anything wrong with it, It is for advanced users that already have a firm understanding of the basics.
The basics of everything to do with vaping, not just ohms law, not just battery safety & selection, not just coil building & wire selection, not just press a button & vape, but how all of it works together and how to get the vape you want by experimenting with and using all of the variables until you find what you like best.
It is not something to set on 100%, mount some coils and expect perfection, you the user has to make all of the adjustments mentioned above. And, you have to know what you're doing & why to know which of the variables needs to be tweaked to get the vape you're looking for.
With your dna250, it does all the calculations & work for you, set the wattage, press button, enjoy vape.
If you want to decrease ramp up using the hexoxhm v3,( how fast the coil(s) in your delivery device (RDA) heat up you have to use a different build.
Resistance is only a small part of ramp up, wire type, wire gauge, coil type, coil mass, heat flux all are factors that have to be taken in consideration and experimented with.
Ramp up = how long it takes coil(s) to reach a satisfactory temperature to provide the vape you want.
Simply put, the 18 wraps of 2) 28/38 you're using has too much metal mass to heat rapidly when using the hexohmv3 fixed max voltage of 6v.
Whereas the dna250 power device is a fully regulated wattage based device, meaning you set the watts you want to power the coils(s) and it supplies whatever voltage is necessary to create the watts you set.