As far as I can tell, it should be possible to roll-back if you save the eMMC state before you attempt the upgrade. But even then things can go pear-shaped, like the eMMC failing at the most inopportune moment…
Well, that certainly is a way forward, if you really can not live with a bit of down-time.
Nope, “brick” is the term for a device that has been rendered non-functional, in your case it might have become superfluous, because the other device has taken over the omnia’s functionality to your satisfaction… Now, even today, a router that offers as much as an omnia is still quite pricy, while a deice for option 1) might be gotten for considerable less than 100EUR (if you are willing to buy second hand probably easily less then 50 EUR).
But your network, your decision. And given that you are not using the omnia as your firewall (IIUC) then sticking to TOS3 seems like a risk you might be willing to accept*. But if all you use the omnia for is as a manageable switch, maybe look at this thread over in the OpenWrt forum, for running OpenWrt on multi-port switches.
Now, without any implies offer to help you getting your configuration implemented under TOS5, I do not believe that a few VLANs will pose an unsurmountable hurdle to configure… It might make some sense though to wait for TOS being based on OpenWrt 20.21, which might include the LuCI GUI for DSA-based switch-ports.