I think this whole issue from a psychological perspective. Just see, even when you were getting enough traffic to your old blog, you left it. This shows that, you were not getting motivation enough with the same old thing.
So, unless you are re-motivated with the same old stuff, there's no point in trying to repair the damaged.
This way, you can enter to the earlier loop and again lose out in traffic and thus your interest.
A custom domain will give you a fresh start, there, as you invest money once, you would feel like working positively towards the blog.
There to make it content rich, you may or may not choose to borrow the contents from your earlier blog, but certainly you would be motivated to get new and fresh content to it everyday.
So from every angle it makes sense to go for a custom domain, and I would suggest from my side, to give it a broader name that can include more topics and subject matter, even if it is on one particular niche.