Well 20 months sounds early for a lot of these to me. My kids are NT
What age your child did most of the beginnings of words. Eg pink not ink, star not far.
a few words by 20 months, but most words later
What age your child said two syllable words.
the odd one at 18 months, but massive change between 18 months and 24
When your child figured out how to pour(water play not anything advanced like serving themselves)
around 2? I think they emptied things before, but pour as in doing it deliberately trying to get it into another container.
Running
well early walker did it early, late walker did it late. Probably was around 22 months for late walker
Climbing steps up by holding the bannister
we lived in upstair flat with concrete stairs, so they had to do this early as I couldn't carry them, but I can remember having to help dc1 when dc2 was a baby, and so he was at least 28 months
Doing scribbles with crayons
this is a lot later than people think. I help in Sunday school creche, and a surprising number of 2 and 3 year olds can't successfully make a mark on paper. And my dc1 was still scribbling at 4.
Using a paintbrush
well, painting rather than just wacking it on paper or stabbing the bristles, again, most 2 and 3s in the Sunday school can't.
Climbing up steps for small slides etc
again, early walker early, late walker later. Can't remember an age, but it was very different.
Tackling steps like pavement drops without going on to hands and knees
hard one to say as I held their hand so they didn't drop to knees outside, (lived in dirty city) about 6 months after walking? so between 16 months and 22 months?
Responding to name(was this consistent all the way through or did you have a period they didn't respond well)
from tiny. literally from weeks old. (but they also chose to ignore me a lot)
Putting on own wellies
really can't remember
Feeding self routinely with a fork or spoon without losing most of the food
well, dc1 used a spoon pretty well by 12 months. But dc2 ate with her fingers until she was about 5. I am not joking, including jelly/soup etc and she hated/couldn't use a spoon.
Drinking from a beaker
With a lid? from about 7 months, open beaker no lid? Again, watching kids in Sunday school creche, that is a lot to do with what they normally drink out of, some kids do it form 12 months, others struggle at 3, becuase they always use a sippy cup at home,
Out of your list, the one that stands out for me is the responding to name, the rest could fall within normal. I know one dd who didn't stand until 20 months and didn't walk until 2+. She had hypermobile joints. I know one dd who didn;t have any words at her 2 year check, but by age 5 she was chattering beautifully.
But if your instinct is saying that putting them together as a whole picture is a worry, then listen to your gut and fight for some help.