To echo what Kane has said - you're not filling your stadium as it is, so you'd have to reduce ticket prices quite a bit to get 100% attendance at a bigger one. With maintenance and other increased costs you might end up worse off financially than you are now. You could work this all out for yourself and see how it looks.
In your position I wouldn't rush to invest in a new stadium unless the alternative is a huge tax bill.
It's a tricky situation as you don't really need top (or even 80+) players to dominate your league, so buying some old stars is just a bit of a waste unless you're using them to push your APR up. And without your old coaches you can't really buy in some quality youth and train them up as they won't reach the potential you want them to. Of course, Firesilver will say you've made your bed
If you really don't want to come back to Serie A I suppose you might be able to put 100 odd points on your rating a season by winning enough to come fourth. Train your youths up as much as you can and do well in the cups, you can probably achieve an APR in the mid-70s which should be enough to get back to 6501.
FTR I was fifth in the top division, had an APR of 74, an ASR of 51, and a club value of 95 M when I reached 6501.