The Overleaf website has a LaTeX template IEEE Transactions. Does it fully support the format as desired by IEEE?
According to Overleaf, yes. The top of their gallery page states:
Here we provide a selection of academic journal templates for articles and papers which automatically format your manuscripts in the style required for submission to that journal.
For what it is worth, the IEEEtran template looks identical to the one that one would download from the IEEE for the Computer Society. Given how easy it is for Overleaf to keep the template file up to date and the loss of users they may suffer from providing invalid templates, there isn't really a strong reason to suspect that they do not have the correct version for use there.
These are the templates tagged as IEEE Official: https://www.overleaf.com/gallery/tagged/ieee-official, but I also recommend to download the latest LaTeX template from IEEE's website and upload it to Overleaf as a zip.