I saw a big fat spider on the wall whenjust as I was just about to start an online English online meeting.

I freaked out, but I couldn't do anything because the meeting was almostbout to starting.

Since then, I haven't seen it, but it should be's probably still somewhere in my house.

I think “should be somewhere” is used to talk about something neutral or positive (“the keys should be on the table”) like you *want* it to be there. So it sounds unnatural if you’re talking about something negative or scary like a spider. It's a bit different from Japanese, because in Japanese when you use はず it doesn't matter whether you're talking about a good or bad thing.

I saw a big fat spider on the wall whenjust as I was just about to start an online English online meeting.

Your original works too, this is just another way to say it, and I think that saying "online" first sounds a little more natural.

I freaked out, but I couldn't do anything because the meeting washad almost startinged.

You could also say "was going to start soon".

Five minutes later, it was still there, but when the meeting finished, it was gone.

Again, the way you wrote it is OK, this is just a little more natural for typing. Since a comma can be used to separate thoughts OR to show where you would pause while speaking, the use can be very different depending on the person and how you want someone to read your sentence.

Since then, I haven't seen it, but it should be somewhere in my house.

Same as my earlier comment, this is just a different way you could use your commas.

A Big, Fat Spider

I saw a big, fat spider on the wall whenjust as I was just about to start an English online meeting.

When using multiple adjectives it is common to separate them by a comma
Rephrasing the sentence like this gives the fact that it happened right as the meeting was about to begin more emphasis

I freaked out, but I couldn't do anything because the meeting was almost starting.

You've already told us the meeting was about to begin so best to avoid repeating that exactly. Phrasing it like this gives more emphasis to the passage of time

I saidthought "Please, don't move until this meeting is over" in my head.

"thought" is generally a more natural choice of words than "said in my head"

Five minutes later, it was still there, butIt was still there five minutes later. However, when the meeting had finished, it was gone.

Rephrasing to reduce the amount of commas.

Since then, I haven't seen it, but it should be since then, but it's likely still somewhere in my house.

We don't usually say that something "should be" somewhere when we consider it being there a negative thing

I saw a big fat spider on the wall whenas I was just about to start an English online meeting.

If you want to use 'when' you need to flip your sentence...."I was just about to start an English online meeting when I saw a spider on the wall'

I freaked out, but I couldn't do anything because the meeting was almostbout to starting.

In my head I said "Please, don't move until this meeting is over" in my head.

It reads better as above

Five minutes later, it was still there, but when the meeting finished, it washad gone.

Since then, I haven't seen it, but it shouldmust be somewhere in my house.

I freaked out, but I couldn't do anything because the meeting was almostbout to starting.

I saidthought to myself, "Please, don't move until this meeting is over" in my head.

Actually, "said ___ in my head" is apparently starting to become popular, but it properly should be "thought to myself", since you don't use your mouth at all.

Five minutes later, it was still there, b. But when the meeting finished, it was already gone.

The sentence was starting to get long, and the contrast looks better after splitting the sentence.