There are a few flaws in this plan
You can't put something into meld into stone
Per the description, and validated here, meld into stone is meant for the caster to walk into the stone, not to create a place you can put another object. The "touch" range for the spell is to touch the mountain, not to touch another creature and allow it to walk into stone.
So the caster can cast meld into stone and carry the now-polymorphed dragon into the mountain with them, but that just means the caster is stuck in the mountain too. Or at least for 8 hours, the duration of the spell.
Meld into stone lasts for only 8 hours
There is no "entombing", as after eight hours, the caster (carrying the dragon) would be cast out. This was talked about in this question. So no matter the condition of the dragon, it would not stay in the mountain.
And I wouldn't call the 6d6 damage that meld to stone causes as "massive"; especially for the cost of a 9th-level and 3rd-level spell. Adult dragons have about 200 hp, so even at max, 36 hp is a drop in the bucket.
However, you can still turn the dragon into an object
So they can try to turn the dragon into an object. But with true polymorph being a 9th-level spell, and the dragon getting three "Get out of Saving Throw Jail Free" uses per day, that's going to be very difficult unless you force it to use up those resistances before even trying to polymorph it.
But trying to entomb the dragon would work better with the mold earth cantrip. Just find some loose dirt, make a 5ft pit, drop them in, and refill the pit. Still wouldn't be that useful, but it's an option.