Would definitely recommend Discord, although it's aimed at gamers, it seems to be an amazing all-round chat service.
Rooms are of an unlimited size and have all the core functionality like file & image sharing, code highlighting, "reactions", which we've come to expect but also focuses more on getting people to speak to each other. It's built on electron and is both really quick & also require almost no resources when running in the background.
Despite having to have Slack for work anyway, I find joining public projects' Slack channels to be too high-commitment, so I never do it. IRC? No problem. I also like that IRC somehow has both a less formal and a lower-BS feel/culture to it.
Might want to try Riot. It uses a protocol called Matrix, which is the successor to IRC. What this means is that you're not tied to them. Also the web, desktop, and mobile clients are pretty sweet.
Does Matrix really have the userbase in place to start calling it an IRC successor so definitively?
It may be aiming for it, and I do hope we see an open chat protocol succeed over all the Slacks and HipChats, but calling it the successor right now seems a little disingenuous!
Don't see anything else around that fits that bill. They also have amazing IRC bridging - I have been using Matrix bridged into Freenode as my primary 'IRC Client' for almost a year now.