The problem with moving it in spring that you have to move it as a clump with as much soil as possible. Fall is best because you can really look at the roots and separate them from the soil and find out what went wrong - may be depth is not right or there is some damage.... May be you just planted it too deep...

I know it is not the optimal time. But I installed a large
decorative gate right in the middle of the garden. The peony is right smack dab
in the center of the foreground. Even if it started growing well this year, it
would be too tall, and no longer work with the design. I don’t want to wait
till fall to get the other low height plantings in.

The other peonies have done so well, that I do blame the
light conditions. Though I don’t recall specific details of this planting, I am
aware of the planting depth issue, as I had a

non-blooming peony at a previous house, 30 years ago, due to not knowing enough at that time.
The ones I planted since 2012 have bloomed.

The big question is that I don’t know how big of a root ball this one will have
(with 4 shoots so far). I will try working around it with a pitchfork first,
and see what I can feel without chopping something off. Like a 1 foot + diameter? 18 inches?

It is hard to kill a peony. Having said that, it will likely suffer some setback moving it now. If you have to move it, move it ASAP. Dig a ready hole, try to get the roots as much as possible and immediately place it in the hole that you have already prepared. Some stems may wilt but don't worry. The crown will produce new eyes by fall.

All good advice. I'll show you some pics that may help. The first picture has had some dirt removed. The second one shows roots that have been washed off. I wouldn't rinse the dirt away in the Spring. This is just to give you an idea of what to expect under the dirt. Keep intact as much as possible. You'll do fine moving and replanting your peony. As gardeners we don't always follow the rules.

Worst case scenario for moving a peony you can assume the root is just as big as the ones that bloomed well. Those roots would be out at least as wide as the plant. I'd assume a 2-3' diameter to start and 18" deep. This should easily be big enough to avoid any roots. You can slowly make the mass smaller by chipping away extra dirt if you don't find roots. If the growth of the root was stunted by the growing conditions it might be less than a foot wide and 6-8" deep. It's hard to predict exactly what you'll see until you start digging, the pitchfork idea is probably a safe bet. Good luck and let us know how it turns out!

H’mmm. The “good” ones were nowhere near
that wide. Maybe because they were smallish bare roots when I bought them. I
don’t remember them being any wider than a foot, maybe 18”, tops. They were
much taller than wide, which to me looked odd, but apparently Festiva Maxima is
a tall variety.