Do you absolutely need to take a specific train at a specific time on one of the peak travel days?

If not, then you will probably be able to make regular seat reservations that meet your needs when you arrive on 18/Dec. The Green pass might give you a couple more options, but with a little flexibility you will get by just fine. (Bear in mind that Green isn't a 100% guarantee of availability either - although it does give you a better chance of getting seat reservations at the peak times).

In a worst case, when there are absolutely no seats to be had and you absolutely must take a specific train, you usually have the option of queuing up on the platform nice-n-early early for a non-reserved seat. Especially at Osaka, where plenty of trains start their journeys, this is a viable option (but realize that there will be many locals with the same idea). Wrap up warm, grab your position on the platform, make a seat out of your rucksack and play cards with your friends until the train opens up for passengers.