The obvious way of building a fuller set of keywords is to retain each keyword, or sometimes a key phrase in practice, and then as new related petitions come along over time, as much as possible, try to stick with the existing keywords as a way of developing a universe of keywords that grows organically. I think that would be the best way of doing it.
My petition on the day of solidarity is going to expire in about a week and it hasn't set off an avalanche of responses, to put it mildly. It could be that in the future someone else will come along with a petition worded somewhat differently. It would be reasonable for a user who's thinking of signing up to look at that petition to find out if there was a previous one that was worded somewhat differently. It would lead to a more informed user prior to their attaching their signature to something. Just as a suggestion, if you could retain that universe and use it for future tags on petitions, I think that would be helpful.