I've had good luck with Keepgo (no affiliation) for low-cost backup eSIMs. Once you add the initial data, it's good "forever" as long as you add something to it once every year. I bought 10GB just over a year ago to have a backup on a different network than my primary plan. Recently added $3 to keep it alive for another year.

https://www.keepgo.com/collections/prepaid-esim-data-bundles