I've been struggling with this, too. I was particularly frustrated by it last night and knew I needed to change the hamburger menu but hadn't really decided how so I put a grey placeholder button that said "START" a la Windows 95. Super ugly so I wouldn't forget to fix it. Except I totally forgot to fix it and pushed it to production. First comment I got this morning was how great it was.
I'm hoping I can find a better solution because I really hate it but at least it's "obvious" now.
I'm hoping a toolbar along the bottom does not work for your app, because I think the Start menu thing is brilliant. Your users have trouble with the hamburger menu that's common in newer UIs, so give them an older UI!
Well I waited all day for someone to tell me how awful it is and... I’m still waiting. So as much as I hate it, the BUB (big ugly button) does indeed live on!
I'm hoping I can find a better solution because I really hate it but at least it's "obvious" now.