Question: The phrase "choose your battles" conceptualises encounters as adversarial.

Any suggestions for less adversarial alternatives?

Answer: 
Distilling the central meanings from the "battles" idiom, the phrases "**save your strength**" or "**don't waste your time**" comes close without invoking confrontation or combat. They could even be used together.

Or, as an alternative, someone might "**consider the cost**" and ask themselves "**Is it worth it?**".

<https://idioms.thefreedictionary.com/don%27t+waste+your+time>

<https://idioms.thefreedictionary.com/count+the+cost>

<https://idioms.thefreedictionary.com/worth+it>
It occurred to me just a few days ago that “choose your battles”
(or, alternatively, [“pick your battles”](https://idioms.thefreedictionary.com/pick+your+battles))
is basically an abbreviated form of [The Serenity Prayer](https://en.wikipedia.org/wiki/Serenity_Prayer),

> 
> God, grant me the serenity to accept the things I cannot change,  
> 
>  Courage to change the things I can,  
> 
>  And wisdom to know the difference.
> 
> 
> 

They are not equivalent, of course. 
The Serenity Prayer advises against
entering into battles that you *cannot* win,
while “choose your battles” more broadly discourages
engaging in conflicts where the risk (or even the cost) is too high
and/or the expected benefit is too low.