By the standards of normal writing practices in Ancient Greek, Mark was considered relatively unsophisticated.
Agreed.
He does the equivalent of starting a lot of sentences with "and", for instance.
So do Matthew and Luke. Compare Mk 15:46-7 (3 times) with Mt 27:59-61 (3 times).
This is one of the reasons why people assume Mark was writing first. It's more likely that later writers would improve his style than Mark would copy more sophisticated writing, but make it worse.
That makes sense, but bear in mind Mark uses colloquial language, with more description and a story-telling style. We need more concrete grounds to discern whether he is expanding his source(s) or others are condensing his account.
The main evidence that Mark is secondary to both Matthew and Luke is the high number of instances where Mark contains words, phrases or sentences that occur in either Matthew or Luke but not both. This cannot be explained as Matthew and Luke taking separate words, phrases or sentences from Mark, because it happens too often.
Here is Mark's account of the feeding of the 5,000, blue = words in common with Matthew, red = words in common with Luke. (Some are not identical but similar to either Mt or Lk). The remainder of the words either occur in all three synoptics or are peculiar to Mark.
And the apostles are gathered together to Jesus, and they related to him all things, what they had done and what they had taught. 31And He said to them, “You yourselves come
apart to a solitary place, and rest a little.” For those coming and those going were many, and not even did they have opportunity to eat.
32And they went away
by the boat into
a solitary place by themselves. 33And many saw them going, and recognized, and ran together there
on foot from all
the cities, and went before them. 34
And having gone out, He saw a great crowd and was moved with compassion toward them, because they were like sheep not having a shepherd.
And He began to teach them many things.35And the
hour already being late, having come to Him,
his disciples were saying, “
The place is desolate, and the
hour already is late. 36Dismiss them, that having gone to the surrounding
region and villages,
they might buy for themselves something to eat.”
37But answering,
He said to them, “You give to them to eat.”
And they say to Him, “
Having gone, shall we buy two hundred denariib of bread and give them to eat?”
38And He says to them, “How many loaves do you have? Go, see.”
And having known, they say, “Five, and two fish.”
39And He commanded them to make them all recline, groups by groups, on the green
grass. 40And they sat down
groups by groups, by hundreds and by
fifties.
41
And having taken the five loaves and the two fish, having looked up to the heaven, He blessed and broke the loaves, and He kept giving them to His disciples, that they might set before them. And He divided the two fish among all.
42And all ate and were satisfied. 43And they took up twelve hand-baskets full of fragments, and also of the fish. 44
And those having eaten of the loaves
were five thousand men.To produce Matthew and Luke's accounts from Mark, they would have had to take the words peculiar to each of them out of Mark independently of each other. This seems less likely than Mark conflating the two.