From my point of view - if I were to offer my opinion on how to manage this situation, you need to focus on a few things:
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For all intent and interest, there needs to be “the one” that will be the captain of the ship. That person needs to have a clear understanding of what they’re doing. That may mean a 3-6 month hiatus readying themselves if they are not already proficient in the said field. They are also going to need a lot of experience with the existing engine to understand how it works - and how a rendering engine works. So far as I am aware this role is still very much open for grabs. Somebody needs to put themselves in this spot. It will not be delegated.
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My management of this situation would not be to modify the existing engine but to build this pipeline independently - as a project in its own right - and then bring in the areas you need from JME as you need them. It strikes me as a far easier way - especially considering a lot of things want refactoring and removing. There are areas that are likely to want to be re-written, too, such as the Post Processing pipeline.
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You need coordination. At the very least a to-do list to remind those invloved routinely. A final outline of exactly what it is you’re doing and what you’re NOT doing. Do NOT discuss on discord. Discuss on the forum where it is well documented and on-point. Nobody needs to read 6 hours of ramble to find a single response.
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Do not forget that you already have a very strong and reliable codebase to work from. Try to understand it’s strengths and weaknesses. It would be madness to ignore it. It’s like a book of the journey you are about to make. Learn.
Start making concrete steps. Make it happen.