In my opinion, as someone who hasn't been playing these instruments very long, the best solution is recorders, whistles, or flutes in general in different keys. Then one can figure out which is the best for a particular purpose. I think the limitation to C and F in commonly available recorders is a disadvantage; the situation with harmonicas is much better.

I use G ones more than any other, C next, F rather rarely. The G is a good fit to the scale of the Highland pipes (transposed to concert A, as pipe tunes are usually played in sessions and by dance bands) - your fingers are often in the same place for the same note as the piper's would be on the chanter.

Unfortunately you don't have many options with G recorders. Susato make altos and sopraninos (actually whistles with the fingerholes drilled in different places) and call them "Renaissance" recorders. They aren't - no recorder ever had a parallel bore. And they don't perform at the extreme top end any better than other parallel-bore whistles, but for pipe tunes they work fine. (I use the sopranino for one tune in particular - "The Hen's March to the Midden", with flutter-tongued chicken noises). They're priced above what schools would want to pay, but at a level where most folkies could afford one.

For wider-range music and a more recorder-like sound you have to spend money. I have a Mollenhauer renaissance G alto modelled after Kynseker which is great for fiddle slow airs, and a very old Hopf renaissance G sopranino made (probably to special order) for Michael Copley of the Cambridge Buskers (he used to blow it with his nose as part of the act). There are several other models of G alto on the market but nothing under 200 quid; G sopraninos have to be hand-made to special order. And nobody makes Baroque G altos except by hand for serious money. There used to be G basses made for German schools - I've never seen one.

Back in the 20s, there was an attempt to use recorders in D and A in Germany. Hindemith's "Plöner Muziktag" trio is written for those. I have an A "sopralto" from that time; it sounds great but I rarely find a use for it. Rather more easily available are descant recorders in B (i.e. C descants made to a low pitch standard). I've got one, made of transparent purple plastic with embedded aluminium glittery bits. If you look round charity shops you might well find one like that. It has exactly the same external dimensions as a normal C descant, the low pitch has been achieved by a different bore profile. It works fine for playing along with singers who capo on the first fret. Surprisingly, it's actually in tune with itself and sounds like a good school plastic descant.

Küng used to make a soprano in B flat (the "Folklora"). It was shit.

But yes, harmonica players have it easier.