I do intend to go beyond Theodore’s death. Presently, my plan is to go at least as far as the 1780s. Once we get into OTL’s French Revolutionary era, however, my job becomes a lot harder, as Napoleon’s absence means a vastly different world history (to say nothing of the fact that the French Revolution itself could conceivably be avoided or significantly altered by various changes in the TL). I may just end the TL somewhere around 1790, or I may decide to keep going with it, depending on how ambitious I’m feeling at that time.
Corsica is not a very good base for unifying Italy, for political ambitions require money and men, and Corsica has little of either. Probably the only way to make it work is through the magic of royal marriage, although the state most likely to achieve unification - Sardinia - has male-only succession. That’s not a problem if you merely want a Neuhoff descendant on the throne; indeed that’s extremely easy, particularly if the Neuhoffs marry into the rather minor Savoyard cadet house of Savoy-Carignano and they end up inheriting Sardinia when the main line dies out as it historically did in 1831. But if you actually want a Neuhoff male-line dynastic king, Salic inheritance makes that tough. IIRC Naples-Sicily is only semi-Salic, so perhaps that’s a better bet if you’re willing to imagine an alternate Neapolitan-led unification. It might be some time before a Bourbon consents to marry a Neuhoff, though.
The other problem, besides the fundamental lack of resources, is that the idea of a personal union is contrary to the founding logic of Theodore’s state. Both historically and ITTL, one of the key arguments for Theodore’s rule - as opposed to giving the crown to an established monarch - was that Corsica, being a small country, would inevitably be neglected if it were a dependency of another, larger kingdom. The Corsicans didn’t have to look far to see how that worked: Despite being “Kings of Sardinia,” the Savoyard kings did not lavish much attention on the poor, backwards island from which they got their royal title. Thus the 1736 constitution prohibits the king from residing outside the kingdom, and it’s hard to imagine a King of Italy (or even Sardinia, for that matter) ruling from Bastia. Certainly constitutions can be overturned, but the Corsicans might not be too happy about “their king” moving off to Turin or Rome and leaving them under some viceroy.
Theodore does indeed possess estates in the Milanese which came to him through Queen Eleonora’s dowry, but it would be incorrect to say that Corsica has those lands. They are the personal property of Theodore, not annexed to the crown. Theodore owns them and collects rents from them, but he is not sovereign over them any more than, say, a Milanese burgher is sovereign over his house. In the Milanese Theodore is just a landowner, not a king.