Should I choose head over heart and purchase a Yamaha instead?
Well, it's not the case that your only options are 1) your grandmother's piano, or 2) a Yamaha. There are a lot of other pianos you might consider. And, choosing a new(er) piano isn't necessarily a choice of "head," it could be seen as a choice of "musical heart" ...
I think it might be better to think of the question as 1) choose the sentimental instrument, which is a piano of unknown quality/sound/touch and which undoubtedly needs a large amount of work with the outcome/success of that work unknowable, versus 2) a piano (used or new) that you select specifically for its quality (in terms of sound/tone/touch etc.).
You could boil this down to a choice between sentimental value and music value. (I hope that doesn't sound harsh!)
I understand the attraction of a family piano with sentimental value. But I think you need to consider whether that instrument will support your musical goals.
A Yamaha Arius is a great instrument to start (or restart) on. It stays in tune and has consistent action across the keyboard.
For anyone wanting to advance their pianist journey, I think the next instrument should always be a step up from the current instrument. So, of course the piano should stay in tune and have consistent action across the keyboard, these are the bare minimum requirements. Then you want a responsive action to help you develop your keyboard technique, an instrument that helps you develop dynamic control, a tone that helps you think about how you play... And you want an instrument that you can have delivered to your home and start playing, not one that needs hours (and thousands of dollars worth) of work first.
The other problem with the sentimental instrument is that you don't know if whatever problems it has will actually be resolvable.
As you can tell, I'm moving in the direction of saying you should get new (or newer) piano because that will likely be the better choice for your musical development.
If you are able to keep the Arius, you might consider getting your grandmother's piano, but you may not have the space to do that? And again, I don't know what your financial situation is, but you could spend a ton of money on the old piano and it might still never been a usable and reliable instrument.
Last question: is the piano the only object of sentimental value you could have from your grandparents? Would you be less interested in the piano if you had something else? What about the piano bench, could you use the piano bench as a keepsake, and get a new piano as a musical tool? (Because that's what instruments are, after all, tools for making music...)
Ok, I'll stop there. I hope this was helpful!