Thank you for your answer, Christophe!
As far as I can see now, there are 3 ways to do this, now just one.
htmlpreview: I used this one mainly due to a very efficient workflow. The link looks directly into the html file in my repo so it updates live with any change I make, so I loved that aspect.
I see, weird as to why that error started coming up now. I have tried it with three of my recent (last month or so) repos and it fails with all of them. The projects that it does still work with, are all from 3+ months ago. I also tried rendering from both a MacBook and Windows laptop and both render outputs have the same issue.
I tried with the default .rmd file but that website only works with .html outputs so even under the best of circumstances, it would not work.
I am not sure about which version I previously used, but the working one would've been older than the current one.
The use case is for sharing publicly, rather than my own use. I find .md files a bit dull to read through for general audiences and lacks the HTML interactivity. Hence, thought folding the code, promoting the use of tabs, kable_extra HTML tables, etc. would make it more user-friendly.
RPubs: just tried this and it works fine for 2 of the HTML files that the former option did not work (I am sure it would work fine with all of them). I guess the main disadvantage here is the workflow. You'd have to reupload each version of the html file as you render them but it wouldn't be the end of the world.
GitHub Pages: I've come across this one before and was thinking to maybe create a centralisee page where I could maintain the various public projects I have. However, as far as I understand GitHub pages (I have 0 experience with them) it's more of a single website that can contain various things (such as pages within that website of each repo)? Rather than a separate HTML preview for each repo. Or is it that both are possible?
Thanks again for your help, very appreciated.