As you write, the motivation is not to maximize the monetization, but rather to solve the issue with the "expected time of delivery", essentially replace the "expected time", which is not working properly, with number in the line, which customers then can see and check, and calculate rough approximation of the "expected time" at the current moment. That I think is a good idea, it should solve the issue.
At the same time, in order to do this, you had to disclose that there in fact already is some form of prioritization. And that I think is a good think too, it's transparency.
Also, as you wrote, you intend to allocate 1 slot per batch (say 5) for prioritization, meaning basically that you limit or control the monetization, so that it doesn't grow out of proportion - it's like you give only 1/5 of the time which can be extra monetized and not more. You could have said, I give 5/5 for extra monetization, but you don't want to do and that is nice - the intention is not to maximize monetization.
So I agree with the intentions, but I'm not sure the technical solution with 2 lines will work properly. It think it will work properly if there is 1 line. If you have 2 lines, which do not correlate, it can happen that people will put the blade to the priority lines, and it may grow so much, that they will end up waiting longer than the normal line ;-). Similarly, if the 2 lines should work so that you put each blade on the "usual" line, and some subset of those will also be on the "priority" line, it can still happen, that there will be so many blades on priority line, that the blade actually will be not made in the priority slot... But perhaps I just didn't understand everything correctly... If not, then there needs to be only 1 line, which is based on both "time of order" and "extra money"... How to correlate time and money, hmm, I didn't think this through yet... Maybe you could say, 1 euro (some amount) per day, where the "time of order" is shifted to the past 1 day, as if it were ordered 1 day sooner ;-)... But here the "control" of the monetization (the 1/5 ratio) is not solved... Anyway, I basically write this as I think - so if I didn't understand the 2 lines properly, this paragraph is to be ignored ;-)