riddle - I have an extravagant sense of humour

Here is my latest riddle, take a crack at it if you dare!

Need to reinforce your brain?

Need help going with the flow?

Then I am exceptional for the job!

I am not a coward, but my family is.

I have an extravagant sense of humour,

I can be within precision, masters, and drastic-ness,

Most people see me and think, “It’s what is on the inside that counts!”

Yet leave out an important part of me!

Well, I am not gonna exaggerate,

But people believe I am an important part of a holiday, yet I am not.

Still people may have me everyday,

Since the beginning of time?

Hint 1

Maybe synonyms?

Hint 2

I am telling the truth I do not EXAGGERATE!

Answer

The answer is

An egg

Need to reinforce your brain?

"Eggs are a good source of several nutrients tied to brain health, including vitamins B6 and B12, folate and choline." source

Need help going with the flow?

(From OP) Eggs contain good cholesterol which helps with blood flow

Then I am exceptional for the job!

If you say "exceptional" out loud it kind of sounds like "egg-ceptional"

I am not a coward, but my family is.

They're all chicken

I have an extravagant sense of humour,

"Eggstravagant"

I can be within precision, masters, and drastic-ness,

Precision = Exact = Eggsact
Masters = Experts = Eggsperts
Drastic = Extreme = Eggstreme

Most people see me and think, “It’s what is on the inside that counts!”

The egg contains a baby chick. Possibly a reference to "Don't count your chicks before they hatch!"

Yet leave out an important part of me!

The shell protects the baby chick. Also (from OP), many people don't like to eat the yolk

Well, I am not gonna exaggerate,

"Eggsaggerate"

But people believe I am an important part of a holiday, yet I am not.

Colored eggs are common around Easter, but Easter is actually a Christian holiday celebrating the resurrection of Jesus and has nothing to do with eggs.

Still people may have me everyday,

Some people eat eggs for breakfast every day.

Since the beginning of time?

A reference to the classic question, "Which came first, the chicken or the egg?"