You cannot upload the letter yourself, therefore you must request it once the portal opens no matter what.
This is a legitimate question, if you believe the letter will be a strong one then asking for it now would capitalize on the fresh timing and authentic wording. Doing later risks them naturally forgetting and losing the specific small/large examples of why they recommend you.
When the letter is formally requested on the portal you have the chance to “waive” your right to view this letter. General advice would be to waive the right to view it. Most OMFS are probably used to receiving a request and entering via portal or coordinating with the applicant at that time rather than giving them a copy months before and then having to upload it again later. (You can still waive your right to view that letter and not feel like you are lying to them because the question isn’t asking if you have already seen it, but obtaining it via formal channels for whatever reason at a later time. There is no easy way to view it even if you did not waive your right.)
All that is to say, I would ask the OMFS this question themselves with your request for recommendation and, explaining your desire to make their life easier down the line by having the letter already completed when the request is placed, what their preference is. I imagine they would think this is a mature and reasonable concern.
If they agree you made a great call and if they decline, then they are probably the type that will make room for it no matter what while also recalling your being considerate of their time.
Edit:
If they are the ones offering you the recommendation now already, I would definitely accept the offer now and explain that you will have to “bother” them again in a few months in order for it to be formally uploaded to your application and that you will let them know at that time when the request is going to be sent and will also include the copy they give you now for reference.
