Yosemite fresh install - an idea I am trying

Question

I Upgraded Yosemite on my MBAIR mid 2013 and Al has worked. I did this as an early adopter the moment it appeared on the App Store.
HOwever ver it does seem slow and dropbox has Ben malfunctioning for three months despite copious support. Other things don't seem right either, like there is a an unfriendly ghost in the machine. So I google that a fresh install while not required may be desirable.
I Find that like all things tech mostly it works and sometimes it does not: other threads on here confirm this; extremes of experience good and bad.
I Have these same experiences. Including time machine backups that even a Genius cannot fix. You can imagine my reaction after al the exhortations to backup to/with time machine. Apparently one faulty folder meant the entire backup was lost.
I have too many tears and years of experience really still to use technology.
So the challenge is how to create a fresh install, to add all my setup, vast number of apps, test it and without resorting to time machine backups ( of which I have FOUR, two external disks and two NAS drives ) plus drop box and additional NAS drive sync (synology drives x 2 about 50 terabytes as one backs up the other) - imagine losing your NAS drive, regret drive two is not offsite, though in a different room)
I Am not convinced my dropbox is accurate nor my NAS sync: one reason for the fresh install.
SOlution I have "in play" is this and interested to hear guidance from experience:
UsB drive, erased with single write of zeroes, partitioned to journaled mac format. Partition about 650gb for MBAIR has 500gB SSD OEM.
Rstart MBAIR pressing command R , follow process choosing EXTERNAL drive to install Yosemite
and after overnight wait for the disk prep process zeroing, now at the point of having installed and asking for my applied. Do I give it or keep it 100% clean while I check the basic install works...... I elect for no sign in..... I want to know if the USB based system works and can reboot. Restarting... I altered password so I can tell which system I have booted from... 70 seconds to the password box....5 seconds from password to main screen...
SO now what? Install apps? So many. The ones I forget will plague me for days or weeks. Or transfer from time machine like its a new machine?
TTime to post this and see if anyone is around with some experience
AApologies for the typing, my iPad is not behaving ideally...

Answer 1

can't edit the above and my ipad would not allow any responses to the post edit dialogues so to continue with what I hope does not become an epic
my name is anthony, please ignore the handle (old long story - maybe one day I can easily fix it)
my first choice was to load mail.app enter my apple i.d., then system prefs icloud and tick the keychain box. There followed repetitive "certificates do not match - choose trust all, around 30 times over once for each email account (I have many). Also enter system password, note to self next time make system password blank for install process. Doing this now for the rest of the process. Yes I am sure (response to a prompt)
So, email has re-installed all (?) of my email accounts with settings (thanks to icloud methinks) saving a huge amount of time compared to past re-installs (thank you icloud) and downloaded 42,000 messages (nearly all imap so as to sync across devices) - I feel a degree of relief already and do not feel that terrible stress feeling when all I have is a backup and no original working system (which is of course still on the MBA's SSD).
When asked about time machine targets: ignore/cancel so as not to interfere with original MBA backups.
Next (concurrently in fact) install Chrome to get back all my bookmarks (thousands). I occasionally try Safari, but struggle because of my time investment in Chrome and it doesn't integrate, imports are not at all the same thing.)
So let's try a restart and compare the timings. ... this is without any apps being reinstalled (as opposed to browser)...