The Steam Deck brings back a memory of a full sized oled keyboard about ten years ago. IIRC it was $1000 and reviews were not great. But the prices of small oleds are gradually coming down and there are a bunch of open source projects out there to build your own, so hopefully in the next few years interesting products that don’t break the bank will show up.
There’s an iPhone app version of Steam Deck, $25/year subscription. It talks to the Steam Deck desktop software, which is a free download, but they don’t explicitly say that you don’t need to have a desktop license to make it work. Probably worth a try.
Or you can roll your own solution via keyboard maestro and a midi controller, either hardware or an ios app (there are a bunch; I haven’t actually tried any yet). The apps give you control surfaces for mac apps. Some are set up specifically for a particular app such as final cut, and some are more open by letting you set up any sort of MIDI control surface. Most seem to connect over your choice of usb, bluetooth or wifi midi. Bluetooth midi is easy, I don’t know if wifi is.
Using midi is of course trivial if you’re talking to an audio program like garage band, but keyboard maestro knows how to do it too. Recognizing a straight keypress/note value is easy, but continuous activation of a button or using a slider look kind of hairy though should be possible in principle.
A hardware controller would have the advantage of tactile feedback. Most connect via usb, but for more money you can find some with bluetooth. A few examples:
iRig pad, $150, 16 pressure sensitive buttons that can have any of three colors, a slider, and two knobs
https://www.bhphotovideo.com/c/product/1056440-REG/ik_multimedia_ip_irig_pads_in_irig_pads_pad_style_midi.html
korg nanokey, $70 (if you just want a lot of buttons)
https://www.bhphotovideo.com/c/product/767243-REG/Korg_NANOKEY2BK_nanoKEY2_Slim_Line_USB.html?sts=pi&pim=Y
korg nanopad, $80, 16 pressure sensitive buttons and one small trackpad
https://www.bhphotovideo.com/c/product/767248-REG/Korg_NANOPAD2WH_nanoPAD2_Slim_Line_USB.html