Bianca V3. I'm grinding with HG1 into Lelit supplied 18-21 g basket. Then WDT with grounds still in HG1 funnel and tap into pf. Takes extra care to manage tapping the pf to sink 19g to point that I can get the tamper into position but still pretty sloppy at that amount.

Current 18.5g shots are quite good but I'd like some flexibility. Alternate basket? Or what?

Less dense and/or darker roasts are going to take up more volume. Are you using a double basket? I know they make triples, this could give you more room.

Different coffee beans and different roast level will yield varying densities, so same 19g will produce a different volume.
Then there is grind size you also have to factor in.

If you want to pull a shot with 19g, then 20g or 22g VST basket may give you a good result as you can normally go finer grind size with it.