He gave you a blank check for a Birkin?

That's wonderful! I'd love to have a husband who would buy me a Birkin.
By the way, maybe he really hates the LP and he wants to replace it at all costs!
Clearly, you love your Loro Piana. So how about you make another deal with him: either a croc/diamond Birkin or a leather Birkin and keep the LP.
I think before you take the H leap you should try carrying a Birkin, you may not love it, as unbelievable as that sounds, or might find it too heavy or impractical. And you need to do tonns of research to know what size, leather and colour combination you might prefer. Bear in mind you may not find the exact one you want, and you'd have to settle for another colour. If you like neutrals, you can go for gold, etoupe, or chocolate brown.