You will need different approaches for different announcements. This is a feature, not a bug. You need to think about how important your announcement is (hint: more important to some people than others), how urgent it is, and to what extent the announcement needs to stick around as reference material. I suggest coming up with 5 or 6 canonical examples. For each one, you will have a subject and an audience, along with the communication channel. Quick, probably not good examples:
- There is no running water on the third floor today. Slack to the people on the third floor, either @ each of them by name or @here in a channel they are all in. Slack without @ in some general/announcements channel that "a number of third-floor people may be working remote today due to water issues, don't be surprised at slower responses"
- Jo Somebody has joined the whatnot team as a Senior Whatnotter. Email. It doesn't matter if this doesn't get read immediately or fully - the important part is in the subject line.
- We have decided what to replace Jira with and you will all be using the new thing starting next week. Mandatory meetings and demos to show people how it works and what conventions your team will be adopting.
- We are closing the Scranton office next week and everyone who works there is moving to the third floor once the water problem is solved. One on one and all-hands meetings for the Scranton people, team meetings for teams that have to share their space or change the way they work, Slack announcement or all-hands email for everyone else just so they know.
For every announcement, consider the consequences of somebody not reading it. If there are none besides a sense of disconnect and not knowing the internal workings of other teams - well welcome to being a slightly bigger company. If there are some, then consider you actually have two announcements, as in the water-off example I led with. Two audiences, two messages. Or three audiences. So be it. Communication is difficult sometimes. Using different media for different messages is, as I already asserted, a feature. A Slack and an email are not the same.
A story from decades ago that happened to me:
We had a small fire in an office where I worked. We evacuated the building very badly. People were taking the elevators up to the 7th floor to get their coats which they had left in their offices during meetings. People were loitering in the lobby, since the building wasn't a raging inferno and it was cold out, and the firefighters literally could not get in to deal with the fire. People were loitering on the sidewalk right outside the building and making little old ladies walk in the slushy road. Once we were finally cleared away from the lobby and the sidewalk, we were kept out until the fire was dealt with. Maybe 20 minutes. And when we got back to our desks, there was a paper letter on every desk from the president of the company chewing us all out for being selfish, lazy, life-risking, fire-fighter-inconveniencing, little-old-lady-bothering nincompoops who should be ashamed of ourselves and would be having monthly fire drills throughout the cold winter until we proved we knew how to do it right. You better believe that the medium was a big part of that message.
If communication is your job, work at it. Don't try to set up one thing that everything goes to regardless of audience, importance, urgency, or need to be looked up later. Tailor your delivery technique to the situation you face. It's work, and it's important work. Do it.