W środku tygodnia jestem bardziej zajęty niż w weekend.

Because Polish usually leaves out subject pronouns when the verb already makes the subject clear.

Jestem already means I am, so ja is not necessary.
You could add ja for emphasis or contrast:

Ja jestem bardziej zajęty niż w weekend.
= I am busier than on the weekend.

But in a neutral sentence, leaving it out is more natural.

Zajęty agrees with the gender of the speaker.

• a man says: jestem zajęty
• a woman says: jestem zajęta

So this sentence is being said by a male speaker. If a woman were saying it, it would be:

W środku tygodnia jestem bardziej zajęta niż w weekend.

This is very common in Polish: even when the verb itself does not show gender in the present tense, adjectives do.

Literally, it means in the middle of the week.

So the phrase suggests midweek, not just any time during the week. It usually means somewhere around the middle part of the week, not Monday and not the weekend.

If you wanted to say during the week / on weekdays more generally, w tygodniu is often more natural.

So:

• w środku tygodnia = in the middle of the week / midweek
• w tygodniu = during the week / on weekdays

Because środek takes the genitive case.

The basic idea is:

• środek czego? = the middle of what?
• tygodnia = of the week

So:

• środek tygodnia = the middle of the week
• w środku tygodnia = in the middle of the week

This is a very common pattern in Polish:

• koniec dnia = the end of the day
• początek roku = the beginning of the year
• środek miasta = the middle/center of the city

Because Polish often forms the comparative with bardziej + adjective, especially with longer adjectives or adjectives like zajęty.

So:

• zajęty = busy
• bardziej zajęty = busier
• najbardziej zajęty = busiest

English often prefers a one-word comparative like busier, but Polish does not always do that. With zajęty, bardziej zajęty is the normal standard form.

Niż means than after a comparative.

So:

• bardziej zajęty niż... = busier than...

It introduces the second part of the comparison.

For example:

• Jestem wyższy niż on. = I am taller than him.
• To jest droższe niż tamto. = This is more expensive than that.

In this sentence, it compares two time situations:

• w środku tygodnia
• w weekend

Usually, no—not in this sentence.

Polish sometimes allows both niż and od after comparatives, but od works best when it is followed by a noun phrase in the genitive:

• Jestem wyższy od brata. = I am taller than my brother.
• Jestem wyższy niż brat. = I am taller than my brother.

But here the comparison is with a time expression:

niż w weekend

That is why niż is the natural choice.

Because w weekend is the normal modern Polish time expression meaning on the weekend / at the weekend.

Even though many nouns after w take the locative, weekend is very often used in this fixed temporal phrase as:

• w weekend

This is the standard everyday form you will hear most often.

Useful related forms:

• w weekend = on the weekend
• w ten weekend = this weekend
• na weekend = for the weekend
• w weekendy = on weekends / at weekends

So even if it looks surprising from a case point of view, w weekend is the form you should learn as a set phrase.

It can sometimes be understood generally from context, especially when contrasting with the week.

In this sentence, it sounds like a general comparison: the speaker is busier midweek than on the weekend.

If you want to make the habitual meaning very explicit, Polish often uses the plural:

W środku tygodnia jestem bardziej zajęty niż w weekendy.
= I’m busier in the middle of the week than on weekends.

So:

• w weekend can work in general contrast
• w weekendy makes the repeated/habitual meaning clearer

Yes, and that version is very natural—but it means something slightly different.

• W środku tygodnia = in the middle of the week / midweek
• W tygodniu = during the week / on weekdays

So:

W tygodniu jestem bardziej zajęty niż w weekend.
means you are busier on weekdays in general than on the weekend.

But:

W środku tygodnia jestem bardziej zajęty niż w weekend.
focuses specifically on the middle part of the week.

Yes. Polish word order is fairly flexible.

The original sentence starts with the time phrase:

W środku tygodnia jestem bardziej zajęty niż w weekend.

This puts some emphasis on when the speaker is busier.

You could also say:

Jestem bardziej zajęty w środku tygodnia niż w weekend.

That is also correct and natural. The meaning stays basically the same, but the focus shifts a little less strongly to the time expression at the beginning.