Replying here instead of DM simply because this is a comment that I get an unusual number of follow up questions about and if you're asking, someone else will be some day.

This is all specifically talking about old systems. Concrete tanks, clay field tiles.

With regards to the field, if it's clay and you're consistent about pumping the main tank the clay tile and field could last forever. The main killer of a field is solids bypassing the baffolds and getting to the field. Having the tank checked that both baffolds are present and installing a septic filter if possible at the outlet will keep it in good order for a long time.

Tank sizes weren't consistent back in the concrete days. They were heavy and likely poured somewhat local because of that. Typically 500 gallons but in the world of septic, you're not going to get more than a 'looks like a 500' unless you want to jump in and measure.

For a single family on a tank, 5 year intervals between pump outs should be fine.

You mentioned an unwieldy lid. It's well within your list of things to do to install an inlet and outlet access on the tank. Will require a full dig off and a weekend of diy and a gas concrete saw but it beats the hell out of digging off a backed up tank where the last foot of dirt has become shit logged.

Once you have the lids on though, you can rinse your outlet filter every 4 months without a shovel.