What constitutes as a restrictive or nonrestrictive?

There are two relevant usages.

The first has a comma before only:

Only = used to show what is the single or main reason why something mentioned in the first part of the sentence cannot be performed or is not completely true:

I'd invite Frances to the party, only (= but I will not because) I don't want her husband to come.

I'd call him myself, only (= but I cannot because) I'm at work all day.

I'd be happy to do it for you, only (= but) don't expect it to be done before next week.

And the second has no preceding comma:

Only = used to refer to something that happens almost immediately after something else:

Cambridge

People were leaving and I'd only just arrived.

We'd only just set off when the car broke down

I leave it to you to decide which applies to your text.

He stopped A, only to B. This is a reasonably common construction in narratives. It does not mean he stopped A for the sole purpose of doing B. It means that he stopped A which was bad in some way, and then began doing B which was also bad.

He stopped running (so I thought I was safe), only to grab a log etc (when I found I wasn't safe).

The tenant in the room above stopped dragging furniture (which was keeping me awake) only to start dancing (which was just as noisy).

The reader is led to think briefly that a problem is solved, but then discovers that a new problem has arisen.

If the comma is left out it would mean that the only reason for stopping A was to B.