Mophilly
that is me precisely. I have played with, hacked on, thoroughly wrecked and (with a lot of help from the nice people here) repaired my SME installs since 2004!
Remember with upgrade to sme9 you are installing a totally clean version of sme9 OS onto clean blank hard disk(s), & all the binaries & file & folder structures are new & as per a default install of standard sme9.
So a lot of what you may have added to sme8 is "not there from the start", eg tweaks, contribs & so on.
Then you restore from your sme 8 backup, which in the main involves data (& config) in the /etc folder & subfolders, which also includes custom templates.
If you have followed recommended sme good practice & only made changes & tweaks via custom templates or user custom templates, then you simply remove all those custom templates either on the sme8 before you do a backup or on the sme9 after restoring the backup (ie delete the whole custom template tree(s).
Then you reinstall any required contribs, making sure they are compatible versions for sme9. If there is a contrib or a few you had on sme8 that you do not want on sme 9, then simply do not reinstall them.
You might want to delete data associated with a contrib if you choose to not reinstall it onto sme9.
It is also a good idea to uninstall contribs from sme8 (that you know you do not want on sme 9) before running the full backup as that will usually remove the data & so it will not be in the backup.