Unfortunately I would agree with the Op and say that it is worth looking further. It may be that something clicks and all becomes "normal" but if it doesn't then early help is invaluable. On their own what she's listed can easily be "normal" but together it's a bigger picture.
What I'd say is:
Not walking at 20 months is unusual. I was always told 18 months was the point that they started noting it as late. Almost all those I knew who were noted at 18 months as not walking, were either walking by 19 months or had a diagnosis of some description.
I did know one bum-shuffler who was about 20 months, apparently they do tend to walk later, but you said he is crawling.
Not all the diagnosis were particularly life effecting, the most common one being hypermobility.
Talking: Well, if he's making animal noises, then he may be within "normal" limits. I think they used to say 6 "appropriate sounds" at 18 months which would include animal sounds.
Does he babble? Does he make eye contact and look like he's having a conversation with you? Is it possible he's trying to say words and not managing to pronounce them? (My dbro had a severe speech impediment where he tried to talk but was totally incomprehensible. I'll add that he probably nowadays would have been diagnosed as high functioning ASD, but, other than a couple of sounds he can't make, you'd never know as an adult)
Can he try to blow bubbles? I don't think any of mine could at that age, but it's good for the formation of the mouth (according to the speech therapist we had!)
Newly eating food: That's an interesting one. I had a cousin who refused all solids until she was about 4yo. I'm talking about totally refusing, including flavoured milk. I remember the celebrations when they finally found a "solid" she was eat. it was only "Smash" but it was a big thing. She is an adult now and has no other issues, in fact once she really started eating you wouldn't have even called her fussy.
My middle one didn't refuse food at 20 months, but was definitely not a keen eater-unless it was chocolate. She refused anything solid in her mouth until about 9 months which was when her big sister put a chocolate button into her mouth. A week later she had a lick of ice cream, and after that did start eating some solids. Reluctantly mostly. At 20 months the only things I could guarantee she would eat was chocolate or tinned sweetcorn. Eating the equivalent of a meal a day would have been good for her. It was often one bite of bread, and a pile of sweetcorn.
So it could be normal, again, I'd look at it as part of a bigger picture.
No meaning behind "mama": Does he say "mamamamama" or "mama" because just the shorter one could be a good sign. It shows that he is trying words rather than babbling.
I'd move on from that and ask what his understanding is like. If you call him by his name, does he respond? If you said to him "fetch me your shoe"/"come here" would he do that?
If he understands, then again very positive.
Also is he developing further? If he's gaining new skills weekly, then that's a good sign. It means he's moving on. If he hasn't really changed anything since 18 months, say, then he is falling further behind.
I'd say from the individual things you've written is that he could suddenly have a burst and catch up. But the fact there's a number of things he's on the low side of development, that makes it less likely-but not impossible.
I'd push for being referred, especially to things like speech therapist, because there can be a big wait. If he doesn't need it by the time he's reached the top of the waiting list, then you can always refuse it.
Remember that diagnosis doesn't change the gorgeous lovely boy you always have had.
It gives you access to help he needs and you can advocate for him to get that. Don't be afraid to push-no one else will.