The walls, if loaded from the inside would be similar with horizontal tension stressed, comparable...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

I've done these. I used the most conservative of a cantilever wall for the vertical reinforcing and and a pinned bottom circular wall for the horizontal. I wouldn't use the rectangular tank approach as the corners are missing.
Since you didn't ask, I would throw away the PCA Book on rectangular tanks (not the one on Circular ones) and just use "Moments and Reactions for Rectangular Tanks" by the US Department of the Interior, Bureau of Reclamation. It's been out of print for about 40 years, but I've found it legally online.

I have also dealt with similar case for some 30 yrs ago.. JedClampett 's approach conservatively O.K. I will suggest some relaxation,

-design for vertical bar assuming cantilever wall both circular and flat wall portions for separate loading cases exterior soil and hydrostatic,

- provide EJ at connections at connection circular to flat wall sections.

- design the horizontal bar for temperature and shrinkage and check for minimum code requirements for the flat walls,

- design the horizontal bar for circular section ; (hoop force (p*H/10)*D/2 + temperature and shrinkage ) and check for minimum code requirements

You may also FEM the system assuming rigid connection of wall to foundation slab and design for the effects.