Thanks I read the write-up for putting together a sitemap and it works, definitely. However (and admittedly if I could maybe code something here, but I am still an amateur, so I will just mention here some thoughts..), the key drawback is the gitlab domain name in place of the custom domain in the sitemap wouldn't be helpful to google search console at least for indexing. I was able to generate a sitemap with the domain name using this took called https://www.xml-sitemaps.com/. So I pasted that into an xml file that I made on gitlab. However it looks like when silex publishes, it overwrites files added manually into gitlab. I remember you had the line that you can edit silexOverwrite:false, however, when I remove the # mark (to make it not a comment?) gitlab throws an error.
Anyways, since my website is very small I can just do things manually just submitting urls one by one, it's no issue in the end. Technically my website is functioning and I've found some clients so I appreciate the free option here