semrush says pages have low text-HTML ratio

I suggest not being ruled by reports generated by various online tools. Use them as a guide not as a checklist where every box needs to be checked off.

So this report is talking about the text to HTML ratio. It is true that a Squarespace page has a lot of HTML to create a page. For pages like events or other short messages there is a very low text to HTML ratio. There is no way around this issue, unless you want to add more text to each page, which generally is not practical for items like an event.