There's two chronological orders in BtW so first formal meeting is subjective to who's PoV you're referring to.

1. MC's Reality (also release order)

2. Mission World (what each man experiences per world)

I assume you're talking specifically about the Cruise World (1st Mission World) here.

By MC's Reality:

1. Main Mission (Boreas or Boreas Copycat)

2. Side Mission (Personal Story) 1 - First Encounter

3. Side Mission 2 - Reunion & Romantic branch

By Mission World or the LI's reality:

1. Side Mission 1 - First Encounter

2. Main Mission (Boreas or Boreas Copycat)

3. Side Mission 2 - Reunion & Romantic

But to throw a spanner in the works, Bai Yuan's Side Mission 2 has MC jumping through time granting his wishes so his timeline is as below:

1. SM2 - 1st Wish

2. SM 1

3. SM2 - 2nd Wish

4. MM - Boreas Copycat

5. SM2 - 3rd Wish

I have a Timeline Flowchart WIP going on. Not sure I'll continue it coz I'm so behind but it does cover the Cruise and Xianxia Worlds.