There are two flavours of 4K on the EM1 II, UHD 3840 x 2160 (24, 25 or 30 fps) with a bit rate of 102 Mbps and DCI 4096 x 2160 (24 fps) with a bit rate of 237 Mbps. Both are of good quality (as opposed to 1080p on that camera), but DCI is visibly better.
DCI will, due to its high frame rate, only work properly with UHS II cards in Slot 1. UHD will probably work well with UHS I cards in either slot. As a safety precaution I would still use Slot 1 for both qualities and Slot 2 as backup and/or for stills.
Although UHS II cards won't improve performance in Slot 2, I prefer to use only one type of cards for my cameras, so that they are interchangeable in case of card failures or lack of cards. You would have to check that the UHS II card actually works in Slot 2 though, but I see no reason why it shouldn't.
Another question is how far you'll get using DCI on a 256GB card. High bit rates eat cards at an incredible speed. I would recommend doing some tests to check the number of GB per minute of video. On an extended trip, I would probably go for UHD, both to save card space and to be able to use both card slots if necessary. Still I would stick with UHS II cards if financially possible, both to have the option of shooting DCI and for future use when upgrading the camera.
4K downsized to 1080 for YouTube (or any other publication) will look much better than footage originally shot at 1080, particularly with the EM1 II which offers somewhat mediocre 1080 quality out of the camera. This is provided you use a good video editing tool (Premiere Pro, DaVinci etc.) for the editing/downsizing. I use DaVinci, which is free from Blackmagic Design.