In this specific case (a list following regular text), this works to shift the list up in Reading View:
.markdown-rendered ul, .markdown-rendered ol {
margin-block-start: -10px;
margin-block-end: 0px;
}
…but this could very well produce unexpected results in other situations, negative margins aren’t good, and I wouldn’t use it myself.
Reading View is going to create lines/spacing between elements. I’d try working with the --p-spacing or --heading-spacing options. For spacing within lists, you could also try the CSS here How to Increase Paragraph Spacing - #2 by ariehen.