How to make communication succinct and more impactful?

I really like this question as I believe it's pretty broadly applicable among software engineers, and in fact, it's something I've struggled a lot with myself.

First, I recommend the following resources on high-quality technical writing:

• "How can I most effectively talk about my projects?"
• "How to get more visibility on work?"

And here's my advice:

• Always have a tldr; for anything >3 sentences - This is covered in the above resources, but it's such an important point that I just had to have it again. This is especially true if you're doing a broader communication like a project update post in Slack/Teams/Email/Workplace. At a big, cutting edge company like Grab, people are very short on time, so this is very necessary.
• Use your messages as a pointer - As a software engineer, it can be very tempting to go in the weeds and break down everything that's happened. This tactic is a way you can have the best of both words. Let's say you have written a project update post. Just have 4-5 lines covering the progress at a high-level (whether it's on track, what was done in the past sprint, upcoming blockers, etc) and for all the nitty-gritty, you can link a technical spec or a similar document (I always have one of these for any substantial project). This way, the people who only care about the high-level details get their concise update (product managers often fall into this category), and folks who want to dig deeper (e.g. other engineers) can click into the linked resource and read to their heart's content.
• Reduce all your communications in length by 50% - This is just an arbitrary goal I've thrown out there, but I've generally seen it work in these situations. Whenever you write something, take the initial length and try to divide it by 2, either through eye-balling or using a word count tool.
• Always have organization - Bullet points are a great start, but I also like bolding/italicizing core points and having clear sections with headers/title. You'll see me do this all the time with my Q&A responses here on Taro 😀 Length is definitely a factor, but perception of length matters a lot too (and is maybe even more important). Even if something is just 1 paragraph, if it has no bolding/bullet points/section, it can feel super long.
• Thank the reader for anything long - As mentioned above, perception is important, and this is another way to help with it: If something is long, people are more likely to feel like reading it was a slog (and hence give you feedback about it) if you aren't appreciative of their effort. Expressing your gratitude is a way to ameliorate that "slog" feeling - You can do this by adding a simple, "Thanks to everyone who took the time to read this!" to any lengthy post/message you write. I highly recommend my video about the power of thanks to learn more about this as well.

Here is how I may restate this question:

My slack messages are too long.
How can I shorten them while retaining detail for PMs?
The only feedback my manager gives me is "Keep it short!"
I now review my messages, but would like to improve.

Often I find value in omission. If somebody wants to know more, they ask.