The only thing that I would see as a concern is how you intend to manage waste until the septic system is installed? True composting toilets are fine and permitted when managed properly, but a pit style outhouse is probably not going to be ok unless it's a sealed vault style.

My neighbors did the same kind of thing (slowly built over 3 years while living on the property) were going to need to a special permit for their toilet and would need to bring the waste to the landfill because they couldn't safely dispose onsite; they ended up renting a porta-potty until they had their septic system in.

Obviously you don't want to contaminate your land and water source. Personally, I would prioritize septic installation above most all else, most people I know who have built or intensely rehabbed focused on that early on, well before contruction/rehab/plumbing.