OS upgrading tips to minimise overhead (rebuilding etc)

[René J.V. Bertin]

Hi,

I was just informed that Firefox will break the sync function in older versions of its browser, including the 78 esr I've been using for the past years. It seems that might finally motivate me to upgrade my MBP to the latest OS it supports officially (10.13). 
(The alternative would be to keep dump OS X as the main OS on that machine and enjoy it under Linux for its remaining lifespan...)

Anyone willing to share tips on how to make this the least invasive and/or disrupting chore, including NOT forcing a rebuild of every port I have installed? Are there any ports that are likely or known to require a rebuild because of changes in system frameworks? Possible candidates I could think of are Qt5 and complex ports like webkit2-gtk .

My /opt/local actually lives on a separate partition which I could make unavailable during the upgrade, if that's a good idea.

A few more open, non-MP-related questions:

I've always done OS upgrades targetting a fresh clone on an external. I currently have a large enough, fast, unused SSD that would be perfect.
Does the 10.13 installer/upgrader convert the boot partition to APFS under any circumstances, something I'd rather avoid as ultimately I'd be cloning the upgraded partition back to my internal disk, an SSHD.

I'll definitely want to disable SIP and all related function aimed at locking me out of my own hardware. Is that something you have to do for every boot disk, and can it be done before starting the upgrade process?

Thanks in advance!
R.