I'd also be interested in knowing. I also feel GitHub's issue tracker is not right for a large project like this.
As for the code review tools though, I've not yet seen something that I feel is materially better than GitHub + PRs + some bot-enabled automation. I understand that many people _prefer_ things like Gerrit, but I've found the difference to mostly be opinion, and that Gerrit can be a significant problem in getting contributions - it's quite hostile on first use.