Suggestion :

Lofting (loosing method) 7 circles to have a profile near of the perpendicular cylinder
cutting the result
adding a cylinder to adjust the central node
I have added a inset to be more near the final result you wanted

Thanks. But the problem I have is it needs to go (in cross section) from a circle to a rectangle and back to a circle again

Bit like this.

Lofting ?

No ?

That works! Thanks! G

And like this :Variable fillets ...so you can put any number of Points and Values! (admissible of course)
Here Red = 1 Green = 3 ... if you have very few place Red = 0.0001 and Green = 3 ;)


Axed generator curve can generally makes problems in Fillets when it is on the same way than fillets's paths!

Else Variable fillets are beautiful simplicity!