I think that to achieve a result easily, you shouldn't try to make half a piece and make a symmetry.
Indeed, I have the impression that if we take the three tentacles on the left, none of the three has the same size or strictly the same shape.
Once you've made your half of the piece, the other half will simply be done with symmetry with the function of the same name.
Why I propose this, because to have the complex shape of the tentacle, you have to make a smoothed base boss with several sketches that give the profile (like slices of sausage) is two or three guide lines that will connect the different profiles to obtain the harmonious curve.
In the attached image, you will see an example of one of the ways to do this, but in your case, you will probably have to make 3D splines given the even more complex shape that a tentacle represents.
I am looking forward to an example (even partial) of @gt22's proposal. Especially to make coherent radii everywhere with the model, since it's radiated everywhere)
With surfaces, he always does simple things that allow us to do complex things. Forttiche what ;-)
I almost go off-topic to thank @gt22 for his example.
Thank you for your example which shows the increased possibilities when you are not able to use the functions of the surface domain. In the example offered by gt22 we get a result much simpler than by the method I used, it's great ;-) (without flattery)
Fortunately I don't make complicated shapes every day (in reality almost never ;-) ) because I would have to get more than seriously into these surface features and if I learned this I would know even more than today and my pov' neurons Ouch !!
