As a guide the adjustment of the registry on a slightly older machine does carry a small risk of corrupting the Win7 which would make it unusable, 5% at a guess. So if this happens then a restore or reload would be needed to recover the broken Win7 setup. This is probably your worst risk. Having copies of your data files will not help this. You would need to repair or reload the O/S from CD/DVD or backup. I am not saying don't do it just beware of this risk.
There is a good tidy up guide here - https://www.wikihow.com/Optimize-Windows-7
I would use the windows backup program built into Win7 and use the external disc if you have space prior to doing this. I would also get the recovery discs to hand if you still have them. I find in life if they are there it seems to go well and not melt down. Sods law.
If you delete the cookies and clean up Chrome then all of your saved passwords and little filling-in that Chrome does will be lost. This means you need all your passwords and user names secure in advance in order to avoid a frustrating day. In my experience the best way of doing this is to use the Windows O/S add/remove programs utility to delete chrome totally. Once the rest of the things are sorted then re-install a clean version from download. This removes all of the plug in issues inside Chrome as well.
Best is to delete all the data and applications that you no longer use. This will remove a lot of stuff that the cleaning can get its teeth into. I would do this first.
Once this is done do the disc tidy to clean up files, this is not really a huge benefit in a modern hard disc unless you have removed a lot of larger files such as video and photos so leaving a mess. Then it costs nothing so is worth doing.
CCleaner is very good and I can recommend it. There is also Auslogics Registry Cleaner Reviews can be found here if it helps. Also links to download them. https://www.techradar.com/news/the-best-free-registry-cleaner-2017
Anything specific come to mind following this ?