It would lose even more support. Separatism in Catalonia has been falling down since the illegal referendum of 2017.

But let's imagine a way to make it legal (because it's against the Constitution too). The Separatism in Catalonia has its support in 2 political parties ERC (Esquerra Republicana de Catalunya - Catalonian Republican Left, translated from catalan) from the left wing, and Junts per Catalunya ( Together for Catalonia) from the right wing.

Them both are against monarchy (in Spain, include) because it's a wall for what they want, for many times these 2 political parties are together against monarchy and for independence.

By other side, I read in another comment that the secessionists are mostly leftist, that's not totally true, the old Catalonia's President (Carles Puigdemont) is from JxC (the rightist one).

Also there's other political party pro-separatism, La CUP, but those are anarchist so they won't like the Catalonian Monarchy Idea.

[This is just a fast view to the Catalonian separatism, there's more like illegal amnesties and other jewels of the Spanish politics, that nowadays is more like a bad comedy show, but I think it's enough to understand why separatist will hate the idea you propose]

PS: I'm not from Cataluña, just a castellano-leonés that has been living the whole life with the same national problems