I have a standing every 2 years on each of my 2 10k gallon tanks. Each year, one gets pumped. That means they both get checked regularly.
Even if it costs $200 for the appointment, it’s worth it to know. They can also likely tell you the size and what they suggest for your situation. They should be able to schedule it for you so you don’t need to remember.
FWIW, ask around to find a reliable, honest septic service provider. Ask your neighbors. Ask your plumber. Ask at the hardware store (if you have an independent or franchise, not a big box. Ask at the plumbing supply store.