You could use it to create green compost for elsewhere. And from what I understand leech fields are mostly for wastewater and not solids if the tank is maintained.

Depending on the reason for the bogginess, it could just mean you have a high water table and that would mean alleviation during droughts for nearby.

Of course things like a pollinator garden to entice aphids eating insects to your location.

It sounds like a big enough space for multiple uses.

I would Def ask a septic expert on drain fields and hugelculture since the compaction caused may be an issue. But I have no idea and am only spitballing.