Well, there are (or in some cases, were) various battleships in service elsewhere in the SFU.
For example, the (Neo-) Tholians had several battleships in service back in the home galaxy, though we don't know yet what kind of production facilities the Tholian Will had set up to build them. While most of them were presumably lost in the Revolt, at least one (the War Havoc) made it all the way to the Draco Dwarf Galaxy, where it was "landed" and dismantled for use as the basis of a new Tholian exile colony.
Plus, there was an incomplete Devastator found at one of the Desecrator starbases the Andromedans had built up (down?) in the Lesser Magellanic Cloud... and the terrifying Devourer variant the Andros fielded in a dark alternate future detailed in SFB Module C3A.
And if you really want to make even a battleship feel inadequate, there is always the Seltorian battlewagon; the very last thing any Tholian ever wishes to see.
Plus, there are a couple of quite large units in other corners of the setting; though getting to those might take quite a while longer, if ever, sadly.