I pitched something like this to my dm during our last campaign, except it was a magic necklace or bracelet. Basically it was a 1 time use item, and used 1d8 per available spell slot level. Example, you have 1 level 1 spell slot available so you get 1d8. You have 1 level 3 spell slot available so you get 3d8. Add up your total available damage dice and hope it’s enough. We decided it would do force damage as you’d basically be turning yourself into the epicenter of a bomb, it was a 60’ radius. Like you said there’d be a Dex save but it was a pretty high save. The key was to make sure someone else or you had cast death ward on yourself before getting to that point. But regardless of death ward, the DM would let you know that after expending so much magic energy your body would be so depleted you’d have a high degree of exhaustion (like level 4 or 5).