Help With Manual Lip Sync

I've recently been working on an animation but I've been having a hard time with lip syncing. The auto lip sync isn't very accurate, I guess because of my accent, but I'm willing to manually do the lip sync if I have to. The only problem is that it's pretty difficult. I can't add/remove a viseme, I can only replace an existing one, and I can shorten a viseme without making the following one longer. Is there any best practice or helpful video/forum post that would help me be able to manually lip sync better? I'm starting to lose hope and might just learn how to use Adobe Animate to lip sync or get Toon Boom Harmony if I can't figure it out.