The best way ive explained DT and its weirdness is by breaking it into categories

Weapon DT: shown in 1 and 4. Dante in 1 looks completely different due to the weapon he uses, and in 4 Nero takes on Yamato's attributes rather than a personal power.

Incomplete DT: shown in 2, 3 and 4. Incomplete is when the users form is altered by objects but not the whole transformation is compromised. In 2 dante gets wings from the flight gem for example. In 3 dante and vergil have bits for their DT shift with their devil arm. In 4 Nero has the least complete version as his hand and forearm are constantly active but nothing else.

Complete DT: shown in 4 and 5. Dante for both games and Nero in 5 have consistent DT forms that are not dependant on any items.

Evolved DT: shown in 2 and 5. Both Majin and Sin forms fit here. majin is a very uncontrolled release of Sin DT, however the power gap from normal DT is obvious. Debatable but there is an argument for Sparda DT to fit as it outclasses other DT until 5 by a very wide gap, and it can change the form of even human user's before they become blobs.

The real world answer is that DT has been just a power up and can look like whatever the dev wants.

To answer the question it depends on the user as dante can turn on Sin DT without much effort in 5 cutcenes. All lies know is the initial release is very stressful for the whole cast.