Maybe the way you've updated the firmware on OFW? Might be a fuse mismatch issue which does result with a black screen. There's an option in hekate to check the number of burnt fuses, somewhere in console info section. If you can boot into CFW on sysNAND or at least semi-stock, it should skip the fuse check and boot into sysNAND. Can't really say as I don't have experience with that. Both of my modded Switches are on firmware that they had at the time of acquisition and I haven't updated ever since, only emuNAND is up-to-date. You can check
this table to see how many fuses need to be burnt in order to boot into sysNAND normally without issue. If you have e.g. 22 burnt fuses but you're on 20.x.x or lower (i.e. lower than expected).
I'm not sure what's the correct approach to fixing this issue but it should be booting into semi-stock and updating the official way, provided that your Switch isn't banned and has access to eShop and everything. Otherwise, you can use Daybreak. Daybreak should be safe to use on sysNAND CFW if I remember correctly reading that SciresM made Daybreak update the console the same way official update does. I'm 99% sure, but don't quote me on that.
I haven't run into a problem on 21.2.0 yet other than a couple of homebrew apps not being up-to-date. 20.5.0 is still recent enough so you can stick to it if you want. You'll have to update eventually, though, so might as well check if everything you're using daily is up-to-date and update to the latest and greatest. emuNAND firmware doesn't really matter, it can be pretty much anything. You need to fix the sysNAND if you want to continue using it, though.