When to switch PhD projects if you are hitting roadblocks
It seems to me that essentially all of the considerations lie in favor of switching projects.  So -- guess what? -- I think you should switch projects.

The one consideration that cuts slightly against this is the lack of funding.  It sounds like when your former advisor departed, he didn't take all his funding with him and there is still some left over to be used by others on the same project?  That's not the greatest situation either: probably the end of the funding cycle does not last as long as it will take you to complete your PhD if you continue working on the project.  It may (or may not be) be that the department can transfer that funding over to your current project.  Anyway, that's not critical because your current supervisor has guaranteed your financial support.  (By the way, if you want to continue in academia than having at least some TA work is probably a good thing.)

Just a couple of random comments:

1) I find it strange that you refer to your advisor as a "they".  That doesn't really make sense: "they" is a plural.  I guess you want to keep your advisor's gender confidential.  I don't really understand why, but okay: that's up to you, of course.  A grammatically correct way to do this is "he or she".

2) You wrote


  I'm making many basic mistakes that would be easily recognized and fixed if my advisor was more knowledgeable on my topic. 


This leaves a bit of a bad taste in my mouth. I have very often had the experience of having students work on things on which I am quite knowledgeable, but nevertheless they often make mistakes.  Sometimes student mistakes are not displayed to the advisor for a very long time.  Sometimes advisors let students make and recognize their own mistakes: that is an important part of the learning process.  If every time a student does something wrong the advisor swoops in and fixes it, the student is not really doing their own work.  [Wait, I did it myself.  Hmm, I think "their" works for a generic, abstract genderless person.  It doesn't work for a specific genderless person...]

A lot of students on this site (and, I believe, elsewhere) are full of theories about how their lack of success (or limited success) is due to some specific external factor that was outside of their control, and that if only they had been placed in an optimal situation they would have succeeded.  That's not so much factually wrong as unhelpfully centered.  No one is in the optimal learning situation all the time.  Learning how to succeed in a real (academic[!!]) world rather than an idealized one is obviously key.  Your mistakes are your own and you should take ownership for them.  Based on your description, your current advisor has been optimally generous and helpful with you.  You should not give off any whiff of a "too bad you didn't have the expertise to help me out the first time" sentiment; that kind of baggage could only negatively impact your continued relationship.