Liza, perhaps start with the tiny bottles of any new liquor, as they are not all great for sipping. In addition to the ones you have, you might consider:
-Amaretto (di Saronno, if you don’t object to the price), great for syruping classic genoise.
-a reasonably good cognac for ganache and for brandied cherries (I haven’t used it in buttercream). The best cognac/brandy I have come across for ganache is actually an Armenian brandy, if you have a local Armenian community you might want to consider it. It’s a hard liquor made from wine.
-if you like the idea of bourbon whipped cream, perhaps a little Maker’s Mark bourbon (wonderful with pecan pie). It’s hard liquor (whiskey), so use less than with a sweet liqueur like Amaretto or Frangelico.
-I also use a fair amount of gold rum for ganache, brioche, gateaux Breton and Basque, and the odd buttercream (pineapple). It’s also hard liquor. My personal preference is for Appleton Estate Gold VX rum, I’ve never been a big fan of Myer’s dark (though it’s a good brand and many people love it).
-Perhaps a bottle of very good quality Kirsch (cherry eau de vie), which goes with many things. If you decide to get one, spring for a good one as the bad/cheap ones are truly awful.
You mentioned Vanilla and Lemon- I have a bottle of vanilla cognac (Navan) and I can’t quite decide if I really like it or not. The balance of cognac/vanilla is pretty heavily slanted towards cognac, and there are definitely some ethereal, floral flavors that are unusual. I haven’t tried it yet with mousseline or genoise. For Lemon, I do like limoncello, but I’m not sure it’s vital to a collection, I don’t use mine very often.
Editing to say, I use brandy/cognac, rum and Grand Marnier the most often, followed by Amaretto and Frangelico. I use limoncello, bourbon and marsala only occaisionally, the marsala is only for Tiramisu, but as it isn’t very expensive but is so very, very good in Rose’s Tiramisu, I would get it if you decide to make that. I haven’t yet sprung for a good bottle of Kirsch, but I should as I would probably use it regularly.