Hi, also Czech here (with an approved CR-1 currently waiting for the NVC to approve documents). I just wanted to recommend to do to the CR-1 instead of K1 if you’re still undecided, especially if you want to be with your partner sooner rather than later, the CR1 is a way to go, also significantly less expensive, and you can still visit on ESTA during the wait (I visited twice since we applied for CR1 and stayed both times 80days)
Also, do start “collecting evidence” of your relationship, take pictures, save receipts, letters, save flight tickets as you’ll need to submit proof of relationship.
Let me know if you have any questions or so, I’d be happy to help 😊