How to define author order of a collaborative paper where all authors have contributed equally?
Since your field does not normally do standard alphabetical ordering, then its use here might signify that something unusual is going on. So you could in principle list everybody in alphabetical order, identify the corresponding author, and then insert a note in the acknowledgments that everyone contributed more or less equally to the paper.

Alternatively, in some fields, papers can be authored by "teams" instead of individuals—although all the individuals participating in the team are cited somewhere. This is more typical when the collaborations run to the hundreds of authors rather than a few to a dozen. If the journal you submit to allows this, it would probably be the fairest option of all.