The DC to dispel true polymorph is always 19
Dispel magic says:
For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a successful check, the spell ends.
The only effect1 of upcasting dispel magic is that dispel magic can automatically end spells of a lower or equal level to the slot used:
When you cast this spell using a spell slot of 4th level or higher, you automatically end the effects of a spell on the target if the spell's level is equal to or less than the level of the slot you used.
Upcasting dispel magic has no effect on the spell DC. Only on the level of spell it can automatically dispel.
The DC only ever depends on the level of the spell you are trying to dispel.
1 - The only effect relevant to this discussion anyways. Of course it has side effects like changing how difficult it is to counterspell for example. (thanks @Slagmoth)