I have to be completely honest here - the Gen4, even with all the reported issues, has grown on us and we’ve been using it exclusively for long-term and everyday viewing and recording. So, we could probably do the Gen4 firmware upgrade on our OG Legacy.
We never really experienced many issues interruptions except for when our network wasn’t optimized. But since we updated our home network devices and cabling, it’s been very good.
With the app provided by @senke7su (which works surprisingly well by the way), the only thing remaining for us would is to be able to manage our recordings/schedule remotely. We don’t really need to watch remotely. Although, this would be a big bonus for many.
After reading through the list of changes in the link provided by @PL1, you do not have to discontinue your Legacy service and you can run them side-by-side, in tandem. I recommend you use the higher-end Roku Ultra 4850 or FireTV Stick 4K Max if you don’t already have one of these. They both have more memory and Wifi6 support.