In this project I updated my own website (slavlee.de) from a TYPO3 11 version in a non-composer based environment to a TYPO3 12 in composer mode.
My website is very simple and is based on my own site package, which is based on t3templates_base. This site package also serves as the basis for all my TYPO3 templates. Additionally, I use some third-party extensions from the TER, including News, Container and Content Defender.
Thanks to my own site package, the update was relatively straightforward. My extensions always use the current standard and the latest core functions. This keeps changes during updates to a minimum.
I have already carried out many TYPO3 updates in the past, so I have developed a good routine and can carry out updates quite quickly. What was new in this project is migrating from a non-composer based installation to a composer based one. I haven't done this often before, but I was able to gain valuable experience.