I really doubt that symptom severity has changed too much across any of the variants.
Omicron is not a milder variant, the reason that it seems milder is because of our immunity. There is more to immunity than what the jabs bring (antibodies). Antibodies give short term protection but there is also longer term immunity from T-cells which aren't fully understood.
Short term immunity is there to provide immediate protection from an infection and vaccine success is only ever measured by whether an infection is prevented or not. Long term immunity however provides long term protection from severe disease and no-one knows for how long, perhaps forever.
The most likely scenario is we've now got a variant which is no different severity wise but it evades past generated antibodies and so we'll see more infections. However, long term immunity stops us from getting seriously ill and so we get the illusion that the variant is milder.
This is perhaps why every virologist will say that viruses tend to get less virulent and less severe, because our bodies learn to deal with COVID and COVID itself will never mutate to the degree required to evade that or it will become completely dysfunctional as a virus.