I'm one of the owners of this group
we have 4,708 members, and since we are using more than 1 GB of disk space, we are paying for the "Premium" package. Our membership is not due for renewal until 30th August this year. What can we expect to pay ( approximately) in 2021, 2022, 2023, 2024 and 2025?
Are we going to be looking at a cost of a couple of hundreds dollars/year, or thousands of dollars per year?
4708 members appear to be in the "Enterprise" membership region, which runs into the thousands of dollars/year, not hundreds, so would be a significant expense.
Perhaps you could clarify more about the legacy groups. If the pricing of our group is going to go anywhere near the thousands of dollars per year, then we will probably look to set up phpBB on a virtual private server and/or use a mailing list. Groups.io suits us well, but I would like to suggest you make it more clear what the plans are for current premium gropus.