Go slow. I think there may possibly be no need for most people to go off the newbie routine after having done it for nigh on 4 months. Having said that, I am just now adding a few other things to my routine only now, and only very very gradually.
There are plenty of variations within the newbie routine. Jelqing at different intensity levels and speeds and different erection levels and with different grips. That takes 3 months or so in anyone’s book. Every time I do it I get new feedback from it. Also the same goes for stretching, I am now doing the manual stretching for slightly longer but with a slightly lower strength than when I first started, but it took me time to work out what was best for me. Also kegeling… short sharp ones? Long lasting ones? Swirling ones? Reverse ones? Where to start… only one way to find out how to do them is to do them. Kegeling is super important as a rock solid smaller dick is better than a big floppy one in my opinion and from being with a few girls, I think they would probably agree.
I would say keep doing it not just until you don’t get any more gains, but also take into consideration your physiological indicators. Erection quality and so on, not just size. Once you do this, then you will know when you need to do anything more advanced. It will probably take you 3 months to even get to read up the theory and start understanding it and how it should fit into your PE.
Good luck with it though.