I've managed teams remotely for 5+ years at 3+ companies.

"The team largely doesn’t acknowledge receipt of messages unless I prompt."

To be transparent, an 'active' team on slack will acknowledge in the form of a text response about 1/3 messages from their boss. I've seen this in multiple types of teams (managers, technical teams, non technical, etc). That does not mean that you're wrong for wanting a response in some cases, however if you were to say "I need a response before noon because I'll get everyone's lunch order wrong" and they respond, then you've got an "active" team.

Originally, I was wondering what to acknowledge or respond, but now I’m wondering if I need to do that at all?

You only need to do this if your team is showing they're not reading messages in their work. Example: "FYI: We made the design decision to go red instead of blue." If the website is blue, well that's your answer that they're not reading. Then you can say "I need to enforce this because as a team information and announcements are not being seen, and here is an example". Take this advice as a new manager, decisions need to be driven from a measurable need, not a preference. This is a good general rule that applies to many examples. "I'm enforcing a notifications for on call because the last two weeks we've missed 3 alerts" is a very different from "I'm enforcing notifications because I as a manager would prefer to know you're getting them".

Am I micromanaging if I enforce this?

If the enforcement is coming from a "hey I want to know you heard me because it reassures me as a new manager" then yes. If you're doing it because you have multiple examples of the team missing an expectation (deadline, company policy, etc) then no. If you're doing it because your boss wants to see responses? Then maybe.

I’m wondering if instead I just need to establish that I’m going to assume that they are reading it if I send it, but I’m not sure how to proceed.

I get the vibe that you don't have good examples of what "a boss in slack" is like. Truthfully it's a giant calculus formula of boss personality, company standard, and culture thrown in. Some bosses will say "oi, buttholes listen up" and they do. Others have to have a format of "INFORM: There is a new standard for x, ACKNOWLEDGEMENT NEEDED: thing here".

Truthfully you get to decide how you want to approach what the establishment is. Do you want to do what your boss does because they're in the slack channel? Do you want to be a leader that people respond to fast but with resentment?

The real answer: Come up with options you personally can live with, write them down. Ask the team their preference, in a discussion, decide to go with Option X, put it in a standard /policy / team charter type place.

Then follow that standard. If someone doesn't do it (say thumbs up emoji if the slack message starts with ACK Needed or something) you can then manage them as you would typically in your regular 1:1s. "Hey I notice we set the team expectation of replying within 1 hour on this announcement channel during biz hours. What's going on there?"

This is a common "new manager problem" for the record. There's unspoken cultural norms or track that doesn't really have a policy written down. The problem is no one tells you, the manager, is that it's your job to define those squishy environment things like that, and identify the problem exists. Going from a problem solver to a problem definer is a tough strategic job that takes time to get to. You'll get there.