When elementaryOS 6 is released, in order to update, do I need to do a full "fresh install" or will I be able to update from Appcenter (or from somewhere else)?
When you're using a common Linux distribution make sure to keep everything important in your personal "home" folder: every kind of personal documents, personal configuration files, applications and scripts you downloaded or wrote yourself, and so on.
This approach makes a sufficient separation between your system and your files, which enables you to backup your personal files just copying your "home" folder somewhere else and then restoring it. Keep in mind that this is a very basic approach though, because there are others backup methods where automations and file system logics are involved.
Most if not all of your UI settings should be saved in
/home/<username>/.config. As the other answers have noted, maintaining a version of your home directory should be sufficient, either by a separate partition, or a backup of the whole folder.
If it is a separate partition, you'll need to inform the installer that there is already an existing
/home partition during the installation process. This way it will simply use the existing files and all.
However, I suggest against migrating your UI settings to the 6.0 install as there are definitely some differences introduced in 6.0.
For me personally, I just backup my working files regularly using a cloud service like pCloud. My settings are not part of the things I backup -- I do this to prevent incompatibilities between current version vs newer version (which was what I did when migrating from Loki to Juno and subsequently Hera).
If I have to work with a separate
/home partition or migrated
/home directory I would remove everything in
/home/<username>/.cache before I install the new OS.
For UI settings, just take 30 minutes after a fresh install to configure it manually. It's a chore but it saves you from having to deal with errors/quirks from incompatible settings, or if the settings behave differently in the newer version.