Inner Tube Tolerances

I have just rebuilt an old bike as a stripped back winter commuter, and as part of the project bought a bog standard mavic 650b (584) wheelset and some pancera tyres @ 584 x 38.

However I seem to have slipped into a gap in the inner tube market where at 38 I am too wide to appeal to the road/gravel market, but not “fat” enough for mtb 27.5 inner tubes.

I can source either 559/622/700 x 38, or 584 x 42/44 in a huge variety of flavours, so have concluded that one of the measurements will have to give by a few mm.

My question to the board is which is more flexible, the size of the tube relative to the diameter of the wheel (i.e. the 559/584/622/700) or the diameter of the tube (38/40/42/44), or does it simply not matter?

During my research I came across Tubulito CX advertised as 650b x 38. On closer inspection Tubulito claim their 700c CX is also suitable for 650b rather than manufacturing them in the two sizes. Appreciating the Tubulito is TPU rather than butyl are inner tubes really this interchangeable? In my line of work 2mm +- is the difference between a bonus or a P45!