So I have a C5 and it was running perfectly for a long time. Over time I started running into issues as things would update and then my rules etc would break. So I tried to stop automatic updates which helped for a while but now so much has broken.
I’m upgrading to a C8 and considering just starting fresh instead of migrating. Most of my rules are in the old rule machine. This will be a big undertaking to repair everything and re-do all my rules but I think it may be best going forward instead of trying to patch old issues.
Can I keep my old rules around for me to refer to and make my new ones based on the old ones? Just set up my new device with a new username etc? Is there any benefit to logging it into my current account?
I'd say it's up to you, but if you're having problems on the C-5, they are likely to come along to your C-8 — a migration is not a magic fix for anything, just way to make the process of moving to a new hub easier (though certain improvements on the C-8 may help, e.g., better range with the external antennas on both protocols and Z-Wave improvements).
If you're considering moving everything to Rule Machine from Rule Machine Legacy, that is also a manual process (though totally optional). However, nothing in any version should spontaneously "break," so I'd figure out why that might be happening if you choose to migrate your existing setup; otherwise, again, that's likely to happen on your new one, too.
You can keep both hubs online at the same time. There is no need to create a new username (My Hubitat/cloud account, I assume you mean?). You can register as many hubs as you want to the same account. I'd suggest different channels for Zigbee (to avoid interference, but check with other things that might be using the same frequency, like Wi-Fi; the hub will automatically choose what looks to be a good channel on first startup, so you might not need to change anything).
And if you do that, you can keep the old hub up to consult apps (including rules, etc.). But you will have to move the devices to the new hub somehow. To do For Zigbee devices, you can normally just reset them and then pair them to the new hub. For Z-Wave, the same generally works, though you could also just go a "general exclusion" using the new hub (not the old one!) before trying to add it, but a device reset also usually clears this. Then, you can reference apps -- including devices they used -- on the old hub while you set up the new one. If manual "migration" will take you a while, you may wish to look at the Hub Mesh feature as a way to share devices from one hub to the other (unless you're able to migrate everything you need for a certain app to the new hub before you'd have to worry about this).
You may also wish to check out the following as you set up the new hub and move devices:
While it is hard to assess the root cause of your problems based on details provided, as @bertabcd1234 suggested above, problems are likely to follow you if you consider migrating. There is merit in starting over when upgrading the controller. Not only gives you the opportunity to build stronger mesh networks, but also it allows you to streamline rules and apps.
With that being said, I am not sure what you meant by stopping automatic updates? One of the biggest advantages that Hubitat offers, is that we don't push updates to users' hubs. We allow our customers to decide when they want to update, if they want to update. We also allow users to revert an update if it causes any problems with existing set-ups.