Help keeping current Belgian employer while moving from France to Germany

Me and my partner are both french citizen currently residing in France (or we’re PACSed at least). We are in the process of both moving permanently to Berlin, and will start actually living there mid-june now that we have the keys to the apartment and that everything is planned.

I’m however stuck on one aspect of the move and that is how to handle my current work. I’m working for a Belgian company, and they handle payroll through an intermediary french company. My understanding is that long term, for me to officially live in Germany and still work for them, they would need to setup the same kind of intermediary than they did in France. However that is quite a convoluted task which requires lots of paperwork they’re not familiar with, and it’s evident that this won’t be setup by the time I start to live there. I was meant to leave them more time to prepare this transition, but we didn’t expect to find a proper non-scam place this fast in Berlin so, kind of suffering from success.

So I’m looking for intermediary solutions for now, to give them and us time to adapt our plans. I’ve found several different types of cross-country work statuses and possibilities but I’m really struggling to wrap my mind around all the criteria and subtleties to settle on an actual solution.

My current understanding would be that I need to:

1. Use the “holiday” work status in a first phase for the very short term. I actually haven’t found any legal text regarding that so I understand it’s not an actual status and more of a legal grey area
2. After that it’s my understanding that my Belgian employer could declare me as a posted worker in Germany. It would required updating my contract, and doing the whole social security handshake paperwork and all that, but it would allow me to keep working in Germany using my French employment contract and social security and such even if I would still need to pay a share of German taxes. But I’m unclear on whether the employer needs to have an actual mission lined up in the target country and if so the degree of proof that is required for that? Can “Settle in Germany” be my mission or does the paperwork involve actually listing clients or projects and such? Also I understand the employer needs to have a presence in the host country but I feel that means my Belgian employer needs to be present in France, which they are not, hence the proxy payroll. So would that not be an option then?

I just can’t seem to wrap my head around the details of it. Which country would I effectively be a resident of, where do I pay/get social security, where do I pay taxes, and so on. If I was just moving and finding a job in Germany it’d be more straightforward, but since it’s my understanding that for the short term I need to keep parts of my French status, it seems more like I’ll have to have a mix and sometimes do things with my French IDs and sometimes German, or sometimes have to pay both governing bodies and so on.

I’m obviously willing to compromise and/or perhaps rest on some of the grey areas of the law, since I’m not ready for this and we thought we’d have more time. If I can find a solution that just buys me some time so neither me nor my employer are in an illegal situation by the time I move it’d be enough for them to setup more things perhaps. I also cannot just go freelance since they’d be my only client and it wouldn’t pass the requirements to not be hidden employment.

Basically I’m lost and confused and unable to hold all the legal points in my mind. I’m mostly basing myself off europa.eu for legal reference since I first followed the subreddit’s wiki, but I’m still unclear despite researching those. Doesn’t help that Europa.eu’s search is kind of really bad.