If “the rest” is singular, as in “the rest of the pie”, then it is “was”.

You need to be careful on what you are speaking (writing) when considering singular/plural.

If it was “the rest of the pieces of pie”, now being plural (pieces), it is “were”.

Similarly, if it was “the rest of the litter of puppies, now being singular (litter), it is “was”

There were two racers. The first crossed the line and the rest was expected soon?

There were many racers. The first crossed the line, and the rest were expected soon.

If it were singular, it would be last, or something similar – The first crossed the line, and the last was expected. (or the other was expected, etc)

Some of the campers went swimming but the rest [of the campers] were expected to clean up the campsite.

After hiking for three hours the rest [here it’s a noun] was expected to occur soon.

I think you have to specify the context. It depends on what the “rest” refers to.

If ‘rest’ refers to a break or a pause between activities, then ‘was’ would be used.

If the story is about a much of people, then “the rest” is plural, and “were” would be correct.

If this is some long complicated story, and “the rest” refers to the rest of the story, then “was” would be correct.

So you would use “were” with “the rest”.