" We were also looking at Barrett outdoor livings insta deck. "

Never heard of them but if something sounds too good to be true it usually is.

Maybe inquire with the septic tank service professional for helpful advice.

I'm wondering how old your system is?

You will, of course, need to leave the access point to the septic tank uncovered by your deck to allow for pumping as you said. That will have to be incorporated into the new deck which shouldn't be a problem. Should you ever need to replace the septic tank any existing decking over it will need to come up too. Ordinarily septic tanks don't need to be replaced (in my experience) as all they really are is a big concrete box. Is someone regularly pumping your tank out now? If so, speak with someone at that company to get their opinion. Good luck!

Yes, we have it pumped every 2 years. Our system is old. But at the last pump they said it could last another 15 years.
I’ll ask them about decking above it!

There are multiple types of septic systems and not all are "big concrete boxes". Many are just large plastic or fiberglass cisterns and any added weight has the potential to damage their successful operation. In most cases, one is instructed to keep the tanks and drain field free of any above ground obstacles other than planting a lawn or other low growing, shallow rooted plantings.

Before you embark on this project, check with your septic system and the company that monitors it to ensure that this is an acceptable practice. Replacing a damaged septic tank or drain field is an expensive proposition....easily more expensive than installing a deck or patio by a significant factor!

An at-grade patio (not deck) should be based on your walk-out basement spaces, access from inside and from the decks, and the greater yard layout. With a patio you can't really cover the tank inconspicuously, so it would be better to avoid it. If you extend the deck(s), then you can have it (somewhat) over the tank because there is plenty of room under the deck to access it (height restrictions would need an access through the deck). You can even reconfigure the deck(s) to have a patio under the deck depending on what's going on with the house. In any case, you do not want the patio, or deck supports, to encroach on the leach field.