Becoming expat in France post Brexit - in need of formalities advice

Hello all, first post here. Little unsure of myself but I'll start by introducing myself and the situation I have.

About me: I'm coming into my late twenties, working as a line manager at a supermarket. Been living on my own for the past 9 months as i moved out of the family home for the first time, which has been very exciting! Studied French at the Open Uni and yet somehow am not completely fluent! It was admittedly a tough course to do fresh out of A levels, with everyone else in the course being retired and had houses/businesses in France and could already speak very well, so it knocked my confidence down to ground level. I have a good enough grasp of the language to get by and, for the past 3 and a half years, I have been in a long distance relationship with a french national whom I met online, so he helps me too. We take it in turns to fly over to each other's country.

We've known we want to live together in France for a little while now but due to our personal circumstances, we haven't been able to do anything about it until now. He has just gotten a transfer to start his new job in Nantes on the 23rd, but he's still finding a flat to live there (he lives near Castres so it's quite a way away). He's got 4 viewings already and has a backup plan if he can't find anywhere before then. As that as a small complication, the following is worse: Brexit, and the increasing likely hood of a no deal. I've been researching visas in prep for that and how to eventually get settled status and I believe I have to first apply for a long stay visa. The plan is that I move over there and begin the long process early to mid December (so I would likely need to apply for my visa before Brexit date has even passed, between mid Sept, and mid October at the latest)

As I intend to apply for the carte de sejour vie privee et familiale, I must get PACSed as a requirement, and also live under the same roof as my boyfriend for a year minimum. Once my visa is within 2 months of expiring, I apply for the titre, which should in theory allow me to stay for another year, and I need to renew afterwards, possibly applying for a multiyear titre (2 or 4 years).
These requirements sound simple enough in theory (it won't be though in practice hah  ), and my boyfriend and I are gathering the documents required for the PACS.
I just have a few questions that I'm not able to find the answers for atm, I've never done anything like this before, so I've joined here to ask for advice from those that may have gone through a similar thing.

The Visa - i can apply for a long stay visa, but I think I would have to apply for a visitor's one, since I am not employed by a company residing in France, nor am I a student. One of the tick box reasons for coming over says marriage, would that be relevant to my situation if I intend to get PACSed? I think the visitor's one is still good for a year but I'm not sure how much validity it has in obtaining a titre. I believe I have to inform French immigration within 3 months of my arrival too, even on a visa, correct?
I've heard I need a tax stamp, which I'm not sure how to get (unless that's part of informing immigration).

PACS - there's a document I need from my country's authority for the PACS that states I am of legal age and capable. Certificat de coutume it's called. Where do I ask for this? I have the rest of the PACS forms, passport and my birth certificate has been ordered, as it just needs to be at most 6 months old. I read on the official service-public.fr that my birth certificate needs to be an original, untranslated document, but then read on a recent blog post that it does need to be translated? I ordered the original but even if i do need a translation I just need to get a french authority to translate it, right?

Titre - Here's the one that's got me most worried. I need to prove that I have had at least one year vie commune with my partner. 3 and a half years of plane reservations isn't quite going to cut it! A little tricky while he hasn't got a place to live just yet either, but I'm confident he'll have a place by late September. Can he start putting my name on the EDF bill, health insurance etc now (not the lease, i don't think it will be necessary), so that I can gain an extra few months time "living with him", if that won't get us into trouble? Or, do I wait until I am officially over there to do that to have proof? Since the visa expires after a year, and I need to prove I've lived with him for that long, but need to apply for the titre 2 months before the year is up... it would look like I only spent 10 months with him. Is there an exceptional circumstance that would still allow me to apply/stay longer to meet the requirements? I'm very confused with this one!

Sorry for the huge wall on my first post! If anyone has any advice or info for me, I really appreciate your time. I'm a huge natural stresser so something like this is sending my nerves into overdrive!!