Ok final updates from the italian sides, we migrated our students and other volunteers from telegram to rocketchat and we are experimenting for the participation stuff.
If the concern is hosting in-house, why not take up the companies behind projects like Mattermost, Rocket.Chat, Zulip and other excellent FOSS chat platforms on their offers of commercially supported hosting of their platforms - that’s the common business model for many such companies fronting FOSS projects like this. Win-win - you don’t have responsibility for hosting, you’re supporting a FOSS project financially, and you always retain the option of moving the whole platform in-house if it makes sense to do so. Unlike with Slack.
The mainly issue is not the hosting but the purpose of using the software and the way. The hosting is the second step
In the Italian case I bought a VPS and we tried RocketChat and we saw many problems of the software (slow interface on mobile, notification issues and too many features so the devs cannot work in what is more important). At the same time we saw issue on moving people from telegram to rocketchat. Indeed we closed that few months ago and moved on telegram with a channel that contain all the list of all the groups and channels (some open other private).
We will see in the future.