One possible alternative to this is to replace Slack with Discord. Discord has no limit on how far you can look back with it. I think it’s more open source friendly https://discordapp.com/open-source, although it’s not open-source itself.
On a meta-level, but less relevant its built with more interesting technology of Elixir and Rust (https://en.wikipedia.org/wiki/Discord_(software)) rather than the PHP that Slack is built on.