I still think that manipulation of environment is a likely scenario, but there are species that have opposable digits and aren’t fully bipedal, so being bipedal isn’t a necessity.
I’ll speculate that in the absence of manipulation of the environment, sophisticated communication might also lead to sentience. Therefore we could expect either very developed vocal organs, or something like rapidly changing skin colors.
So we’re still looking at monkeys and squids.
The problem for an intelligent sessile species is, what does it use its brain for? And on Earth there aren’t any completely sessile land animals because sessile animals are invariably filter feeders, often with a side of symbiotic algae living inside their tissues. There are animals like spiders and ant-lions that construct traps and wait for prey to wander in, but even then they aren’t sessile.
As for radial symmetry, it’s pretty rare for complex animals. All the radially symetrical animals I can think of are slow moving echinoderms and coelenterates, many of them are also sessile.
The reason so many creatures have necks is that a neck allows you to move your mouth and sense organs around without having to move your whole body. So you can grab your food more effectively, you can find your food via your sense organs, and locate predators.
An animal has to eat, it has to protect itself from predators, and it has to reproduce. Sensory suites are vulnerable and require a lot of brains to process effectively. Most animals on Earth only have enough brains to find just enough food and avoid predators often enough to reproduce some of the time.