You need to use the first three characters to defeat the boss after the heart, which is a prerequisite for unlocking the true ending.

how do you get to the boss after the heart?

Just succeeded, so I'm noting this down for future people. Used Shion's second skin, stacked strength up to absurd levels and managed to knock down the worm to it's weakened phase. From there, I conveniently had a Mors Universi from the card in the wall event and ended up tanking the boss until they got judged. Afterwards, it said I had completed the true ending.

To elaborate further, that run was a miracle in itself. If it wasn't for the fact that I had a bunch of gain ethereal cards, I would have just died.

Im struggling to unlock the true ending, ive beat the boss after the heart at least once on every character and almost on every 'skin'. the option true ending option is still greyed out and wants me to defeat that boss after the heart with some listed characters.

I think the mod is bugged. I've beaten the boss after the heart with every character and alternate skin I could unlock, and still nothing. The option is still greyed out.
Do I have to beath them on ascension 20?

For the boss after the heart, is it the worm guy, or the lady on the bike? because I'm having trouble unlocking the skin where i need to get the "true ending"

true ending is beating the worm