Hmmmm? Let’s see. You listed, what, nine whistles in a month on another thread. Right? You’re on a trajectory for 80 some whistles in a year.
So, get it over with. You will eventually buy a whistle (or two or three) in every key made. So you may as well get on with it.
From what you have mentioned, you’ve got whistle you like in D, C and Bb. So here’s the plan.
Get the rest of the Generation family - high G, F, C and Eb. Might as well. Everyone has to have these - even just to complain about them.
Then get a low A and a B natural. Follow those with the low G and the low F.
After a bit you could look for a low E and then the Eb.
Once comfortable with those, see if you can get that Optima back. It is really a decent low D whistle, especially for a new low whistle player.
Then you can get the low C and maybe fill in on the flat keys.
Resistance is futile.
Suggestions of a maker? Hmmm? Depends on what playing characteristics and timbre you prefer. Any whistle I suggest would likely not suit you. Same might be true for anyone else’s suggestions too. It’s all relative to our own preferences, experience and skills.
Besides my own homemade whistles, I’m playing Burkes in all the keys (that I need), and in all the different materials (but not in all keys). It’s only a couple dozen whistles or so. But they are solid, tuned well and share the exact same playing characteristics across the board. No surprises from key to key. Or maybe Goldies, Brackers, etc., etc., etc.. Lots of very good whistles out there.
Feadoggie