I think you've gone about this the wrong way, no matter how well-intentioned.
You decide how much money you want to give someone, then you give it to them. If you can't trust them to deal with a large lump sum, then either they have some kind of issue (like drugs or gambling) that means you shouldn't give them any money at all, or you want to impose conditions (so it's not really a gift, and shouldn't be given). Gifts with conditions rarely lead to positive outcomes.
You can't control your DC's choice of partners, and it's unreasonable to ask someone to keep secrets from their partner, especially where it concerns a large amount of money. You might view your family unit as you plus your DC, but for your DC, that family unit is widened to include their own partner, and likely even partner's family.
You should never lend money you expect to see back - a lesson you've learned the hard way. No need to balance it out by giving more money to the others. You've apparently gone no contact, which indicates you didn't favour that DC more than the others, so any sense of injustice your other DC feel should be against that sibling and not their DM.
If you're so worried about inequality, maybe update your will to state that any loans with DC should be taken into account before divvying up your estate? (Assuming, of course, you're planning to leave anything of value and won't spend it all by the time you pass away.)
I wouldn't make any further gifts of money to family. It's clear that it hasn't brought anyone any happiness, so better to shut shop and move on.
How much money do you have left? A small amount, or still plenty to make a difference? If you have more than you need, a small gift to a local and overlooked charity would result in a genuine, warm thank you and that might be the balm you need right now after not getting the reaction from your adult DC you had hoped for.