Obsidian doesn’t have a web app. The sync service works by keeping the local files sync’d with the files on the server. I’m not sure whether a web app is planned in the future.
On the MacOS desktop VeraCrypt could work as long as you dismount the encrypted container whenever you are not using it (especially if you use VeraCrypt’s auto-dismount features). You would need to make sure Spotlight doesn’t index the encrypted drive when it is mounted by VeraCrypt. I’m not sure how well this translates to iOS devices.
You may want to take a look at Standard Notes, Notesnook, and other privacy centric note taking tools. It looks like Standard Notes and Notesnook encrypt files before they save it to your computer, but I have not personally used either to verify. I’ve been burned by other note taking tools in the past, so I would encourage you to test them out, make sure you can easily export your data in the future if you ever want to, etc.