I honestly dont really know many o2 servers. Maybe o2v3 is what you're looking for, but I don't have a link for them.
As for my converts, many of them are indeed offtime. Because the o2jam chart editor tools were not meant to be used for making charts to existing songs, it's extremely difficult to time things right in o2jam, so many of the unofficial charts are offtime. I didn't fix it because sometimes fixing timing in osu's editor can be a gigantic pain in the ass, and I had to manually convert every chart, which took several hours of "press the convert button, wait for conversion to finish, select next song, repeat". Occasionally I also had to manually extract the background and add it to the osz. (not to mention the 42 hours I spent uploading everything to mediafire). I just gave up on fixing any problems with them.
One thing to note about o2jam is that it has very large timing windows compared to osu!mania, so practicing there will not be really helpful for improving accuracy at all. On the other hand, missing on LNs is extremely punishing, and there's no way to re-press them if you accidentally release the LN too soon, which teaches you to avoid mistakes on LNs.