Same! I use Feedbin (https://feedbin.com/) to aggregate new posts. On desktop, I'll sync Feedbin up with NetNewsWire (https://netnewswire.com/).

Substack and Medium both support RSS. You can just type /feed after the URL. e.g., tk.substack.com/feed and tk.medium.com/feed

For other newsletters, I use Kill the Newsletter (https://kill-the-newsletter.com/) to subscribe via RSS.