Repurposing a SharePoint "Send a reminder" flow 

There is a "Mail" connector with just one action ("Send an email notification (v#)" - the # was 3 the last time I used it, not sure if it's been updated since) that sends from a Microsoft-managed service account. If there's a chance people will respond to the email, you might want to look into using a shared mailbox. There's no license cost for a shared mailbox, you just need your M365 Exchange admins to create it for you (unless you are an Exchange admin).

You need to more clearly define what you want to do. It sounds like maybe you mean every day at 9:00 you want an email sent to the "wealthmanager" (I'm not sure what that means?) listing anything added or created in the previous 24 hours? Is that correct? If so, you just need a daily recurring flow that gets items from the list or library where Created is greater than or equal to the current date and time minus 24 hours.

Yes, this is possible. Basically, you would have a flow that runs once a month and retrieves all items with a "reminder date" that's within the desired time frame. Then, from that data, you need to get a unique list of "owners". Then, for each unique owner, get "their" documents from the initial list and send them an email. So, this entirely do-able, but involves a bunch of data manipulation. This video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-afqvGAb20Dw.htmlsi=yjnMhdAbv5ofph5O would be a good primer on how to do some of that manipulation.

If you're not getting the email, then I would suspect there's something wrong with your flow. It's difficult to troubleshoot these issues through RU-vid comments. I recommend you post a message in the Power Automate Community (community.powerplatform.com/forums/thread/?groupid=46ce02a3-e1a7-4176-81fc-d93a4001d287) with some screenshots of how your flow is configured and an explanation of what you're trying to do.