I had the exact same idea, word for word. It's a great idea, but the issues that I realized exist in it are the following:

1. Necrotic resistance breaks the spell.
2. The spell effectively doubles your healing power when you have a minute to keep healing yourself, and transferring that health to your allies.
3. The player can heal themselves if they roll high damage, and retain their spell slots if they roll low, effectively turning their preparedness for new encounters into a random chance rather than choice, which is unsatisfying, and makes planning balanced encounters as the DM more difficult.
4. The random chance that you'll end up taking more damage than you heal is less satisfying in practice than it sounds in theory.
5. No other spell in 5e can accidentally kill yourself based on the results of a die roll, because that shit is as heartbreakingly lame in practice as when it happened in Critical Role.
6. You could transfer the Necrotic damage off of yourself onto another player with Warding Bond, halving the damage with resistance to all damage. It's not the worst downside, but it negates the cost of this spell pretty effectively when the damage is divided between two players. If the damage is an odd number, it rounds down per 5e rules. If it is 1, it becomes 0 for each player. If you have two players with Warding Bond on the player the damage is divided by half, and then half again, drastically reducing the damage taken and increasing the odds of reducing the damage to 0. This just becomes more potent with the addition of more Warding Bonds stacking up until there is 0 chance of the castor taking any damage. Players in possession of a Shield Guardian also get access to this exploit, and then the players can repair the Shield Guardian through one of multiple somewhat  mechanically vague means.
   
   

I have a solution with a completely different working healing cantrip, but I try to make a little side money with my labor as a creator, so I sell my work on DMs Guild rather than give it out for free on DND Beyond, or else I'd just give it out here in the comments.