Tl;Dr: It's not possible using built-in features only.
Reddit regular users are not able to schedule posts using the built-in features. The Reddit User Help Center doesn't include any mention of "scheduling posts" (tried several word variants). The article How do I post on Reddit? doesn't mention how to schedule a post. There is not help article about how to add a comment.
Reddit moderators are able to schedule posts. For details see Scheduled and recurring posts. This help article doesn't mention that comments could be scheduled.
You might use third party apps for Reddit or apps like Zappier, Make.com, etc.
For those with programming skills or interest, an interesting thing to consider is to join the Reddit Developer Platform. See Coming Soon: Reddit Developer Platform. On the sign up form for the waiting list you will be asked what do you want to develop. You might mention that you want ot develop an app to schedule comments.
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