I think you should keep it running.
My suggestion is keep it public, but limit the volume each person can write to it without paying.
First thousand notes free etc. something like that.
Options:
1when hit the note limit Pay one off fee to archive older notes and reset limit or they will be pruned
2pay much higher one off fee for increased limit
3subscribe to remove limit all together+ extra features
Damus is definitely valuable enough for you to cover costs.
