The Vero 4K(+) cannot pass through 1080i video - it can produce 1080i output, but this is only available for displays that are so old they don’t support 1080p input, and it’s produced by deinterlacing and then reinterlacing again. (The same applies to 480i and 576i output - they’re only available for TVs that can’t accept progressive input, and are not “pass through”).
The choppiness happens because it will deinterlace (say) 1080i/50 to 1080p/50 in a way that (incorrectly) treats the video file as video mode instead of film mode, and then converts that to 1080p/25 by dropping frames. You can work around that by de-whitelisting 25Hz, 29.97Hz and 30Hz modes so that it outputs 1080p/50 or 1080p/60 instead - that fixes the choppiness, but you’re still seeing video mode deinterlacing with consequent loss of resolution. (If the video happens to use VC-1 codec you can also manually force a “weave” deinterlace and get perfect playback, but that doesn’t work for MPEG-2 or h.264).
Behaviour with no whitelist is very different from behaviour with everything whitelisted. In particular, with no whitelist, the resolution can only switch up and not down - so SD material will be output as 1080p, for example. And also it will only switch if the video resolution exactly matches an output mode; so 2560x1440 will be downscaled to 1080p instead of upscaled to 4K.
I wrote a whitelist guide a while back… it needs a little more work with respect to new features in Kodi v19, but you might find it useful: