Мне лучше бы было взять с собой термос с какао, потому что на остановке я чуть не замёрз.

Because this is an impersonal construction.

In Russian, ideas like it would be better for me to... are often expressed with:

• мне = for me / to me (dative)
• лучше = better
• infinitive verb = the action

So:

• Мне лучше бы было взять...
  literally feels like
• For me, it would have been better to take...

Russian does not say this as Я лучше бы было... because я would be the subject, and this structure does not really have a normal subject.

This combination expresses regret about a better action that was not taken.

It breaks down like this:

• лучше = better
• бы = would
• было = was / would have been
• взять = to take

Together, it means something like:

• It would have been better to take...
• I should have taken...

So the speaker is looking back and saying that taking a thermos would have been the better choice.

It can look repetitive to an English speaker, but in Russian they do different jobs.

• бы is the particle that creates a hypothetical / conditional meaning.
• было is the past-tense form used in this impersonal structure.

So:

• лучше взять = better to take
• лучше было взять = it was better to take
• лучше бы было взять = it would have been better to take

In modern Russian, speakers often prefer:

• Мне лучше было бы взять...

That word order is often more natural, but Мне лучше бы было взять... is understandable and grammatical.

Because взять is perfective.

Here the speaker means a single completed action:

• to take a thermos along on that occasion

Russian often uses the perfective infinitive after лучше / надо / стоит when talking about a specific action that should be done or should have been done.

Compare:

• брать = to take, in a general/process sense
• взять = to take, as one completed act

So лучше бы было взять means it would have been better to take on that specific occasion.

С собой means with oneself / along with you.

So:

• взять с собой = to take with you / bring along

This is a very common Russian expression.
Examples:

• Я взял с собой воду. = I took water with me.
• Возьми с собой зонт. = Take an umbrella with you.

In your sentence:

• взять с собой термос = to take a thermos with me

They are two different constructions.

1. с собой = a fixed expression meaning with oneself / along
2. термос с какао = a thermos with cocoa / a thermos containing cocoa

So:

• взять с собой термос с какао

means:

• to take along a thermos of cocoa

The first с tells you how/with whom it is taken.
The second с describes what kind of thermos it is.

Because какао is usually indeclinable in modern Russian.

That means it often keeps the same form in different cases.

So after с in the meaning with, you would normally expect the instrumental case, but какао stays какао.

Examples:

• люблю какао
• нет какао
• с какао

The form does not change.

Russian uses на остановке as the normal way to say at the bus stop / at the stop.

This is just the standard preposition used with остановка in this meaning.

• на остановке = at the stop
• на автобусной остановке = at the bus stop

To an English speaker, на may feel like on, but prepositions often do not match one-for-one across languages.

Чуть не means almost, nearly, or I almost did X.

So:

• я чуть не замёрз = I almost froze

This expression is very common in Russian.

Examples:

• Я чуть не упал. = I almost fell.
• Она чуть не заплакала. = She almost cried.
• Мы чуть не опоздали. = We almost were late.

It often appears with a past-tense verb and refers to something that nearly happened but did not actually happen.

Because замёрзнуть / замёрз is perfective and means to freeze / get very cold to the point of freezing.

After чуть не, Russian often uses a perfective past verb to describe an event that nearly happened:

• чуть не упал
• чуть не умер
• чуть не замёрз

By contrast:

• мёрз means was cold / was freezing as an ongoing state or process

So:

• я мёрз на остановке = I was cold at the stop
• я чуть не замёрз на остановке = I almost froze at the stop

Not exactly with an explicit if, but it expresses the same kind of unreal past regret.

Russian often uses бы constructions without a separate if-clause when the context already makes the regret clear.

So:

• Мне лучше бы было взять с собой термос...

is basically:

• I should have taken a thermos with me
• It would have been better if I had taken a thermos with me

Russian does not need to spell out the if here.

Yes. Russian word order is flexible, and several versions are possible.

For example:

• Мне лучше бы было взять с собой термос с какао...
• Мне лучше было бы взять с собой термос с какао...

The second version often sounds more natural to many speakers.

The meaning stays essentially the same, but word order can affect rhythm, emphasis, or style more than core meaning.

Yes. Потому что is the most common neutral way to say because.

So:

• ..., потому что на остановке я чуть не замёрз. = ..., because I almost froze at the stop.

Other options exist, such as так как, but потому что is the most everyday and straightforward choice here.

Because this is an impersonal sentence.

In Russian, impersonal past-tense constructions often use the default form:

• было

This does not have to agree with a visible noun here. It functions as the standard past form in this kind of expression.

Compare:

• Было холодно. = It was cold.
• Мне было трудно. = It was hard for me.
• Мне лучше было бы взять... = It would have been better for me to take...

So было is there because the whole statement is built as an impersonal it-was-better type construction.

Yes. That is a very natural English translation.

Even though the Russian structure literally looks more like:

• It would have been better for me to take...

the most natural English rendering is often:

• I should have taken a thermos of cocoa with me, because I almost froze at the bus stop.

This is a good example of how you should not translate Russian word-for-word when a more natural English equivalent exists.