It depends on the area of Linguistics you are interested in - if it's an area which a large Psycholinguistics component, your current background may be enough. Tbh, Humanities PhDs are competitive funding-wise, so a lot of research councils ask for specific previous degrees - you might want to research potential supervisors and funding sources to see whether the route is viable for you.

There is also the MA+PhD route (1+3), which would fund a relevant masters degree plus the PhD, which might be suitable for you if you have a less than ideal academic background for Linguistics.

I agree with Tudor_Queen. Research assistants who are fluent in the languages you're analysing would be best, as translation software is never that reliable or accurate.

It depends on the field, but some research councils will not fund current PhD students, only students will are yet to start their project.
Also, I'm not sure what you mean by looking for a fully-funded project for next year. If you've started a PhD elsewhere, you may find that it's frowned upon to up and leave after 1 year, and is a bit of a waste of time and money honestly.

Your mindset seems pretty persistent, so I think seeking mental health support is very important at this stage. If you need to apply for an extension in order to complete your project satisfactorily, then look into doing that. If you've had to restart your project in your 3rd year, that definitely needs addressing - by your committee, and by the university.

I think a big question for you is: what would make you feel less incompetent? And how could you go about achieving that?

I can relate, not at PhD level, but some things to do with my undergraduate degree can give me a very visceral response. Like you, I received very little support during my undergrad (even when I actively sought it), and felt like I wasn't wanted. My particular course had a small cohort, so it was easy for people to stand out for the right or wrong reasons. Only a few weeks ago I was reading an academic paper about a certain topic highly relevant to my undergrad, and I found that I suddenly felt very tearful and panicky. I graduated in 2014, so I was surprised to still feel like that.

What helped me was making sure that every career decision I made from then on came from a place of seeking happiness overall, and not doing anything as a means to an end. It also gave me a good measure of who are good people for me and who aren't, and to trust my gut instinct more.

Everyone NEEDS constructive feedback, even if it's not necessarily something you WANT to hear. I always try to only look feedback when I'm not tired and in a reasonably pleasant mood, that way I am more able to appreciate what is being said and why. I also think that you need the mindset of: would I have noticed the need for these changes if my supervisor hadn't pointed them out? Maybe not, as we are all guilty of being 'too close' to our project/writing, and an unfamiliar reader can help with that.

Comparison is the thief of happiness, so you won't make yourself more productive/generally better by doing that. Also, everyone's PhD structure/trajectory/process is different, even those in the same discipline, so there is literally no point in comparing yourself to where others are and how they are doing.

I don't understand how you don't know what your topic is; just to clarify, did you apply to a pre-proposed project or did you propose you of your own?

As your supervisor is new to the position, I'd say that you can't be backward about coming forward - make sure he knows what you want from him and then ask for his expectations too, then you can negotiate if they don't align. If I were you, I'd also strongly request a second supervisor.