I agree in general with TinHead88.
My experience over the last 30 years using, then fixing, Macs is this:
the cleaner the install, the cleaner the performance (PERIOD).
I used to do a CLEAN install: manual file migration, all preference files recreated... each time I did an OS upgrade starting from 10.3.9 to 10.4.x.
I used to wait until 10.x.3 service release came out for any os.
I used to keep my main machine TWO years / versions behind the current Apple one, as it seemed from all reports, and all the work I was doing on other's computers that this was highly beneficial for productivity unless there was some exceedingly important reason to upgrade, i.e. user needed latest version of X software of current version.
Sometimes undesirable behavior can be linked to a single corrupted preference file.
Then, later, when I decided 10hours of active time, and an entire weekend to get 90% of my Mac migrated over by hand and all preferences reset / recreated how I had them, etc. was too much of a pain in the A, and migration assistant had SOMETIMES worked without a hitch, I would use it every >other< time I upgraded macOS/X, and the alternating upgrade of the OS I would still do a clean / manual upgrade/migration.
For a while, one of my clients was a small school with about 20 Macs, I would refuse to upgrade more than one OS version at a time, I would not skip a single one, and insist that even in place OS upgrades would have to be used for a couple months, and if the users still couldn't work with that, then I would upgrade again, but only after things like mail, photos, etc. were allowed to be launched at least once and databases migrated/updated to the newer versions as is done on first launch after an os upgrade. Why? Because sometimes >skipping< an OS by upgrading from say, 10.10 to 10.12 would result in nasty results.
There are many more technical examples I could waste my time typing here, but if you don't get what those of us highly advising you to do a manual migration from, just a guess, 10.11? to 10.15 (four OS versions), you may get undesirable results, you may watch your Mac work on entire day on upgrading your iPhoto library to a "Photos" library... you may get antsy while Mail upgrades things, only to fail for some reason, or decide it's not working, and force quit or reboot.
You may, get off without a hitch. This is theoretically possible, and for some us, we are destined not to be in that group, hence our advice, for others, who might be professional programmers, they will say "use Migration Assistant! Otherwise you might forget/lose something in manual migration." Which is a perfectly valid argument in another way. Or "I've never had a problem, so why would anyone else?" Which is a little less sage advice, I hope you can see, one person's anecdote, is not another's reality. If computers worked perfectly, they would have been designed by computers. We're not there yet. As Apple continues to push its desktop OS toward its mobile one, more and more faith has to be placed in them, as more and more of our power tools are taken away from us or "deprecated" with the only justification being more control by them, and less by us. Unfortunately, computers are inherently flawed, just like their creators. After decades of troubleshooting them, I can say, programs do not behave as they are "supposed" to, including one called migration assistant, just because they were designed to. I have been able to intuit solutions to problems that programmers with far more experience and understanding of computers could not. Why? Because the programmers want a "reason". With a hundred (user facing) programs or more, the average computer's behavior cannot be explained in a logical manner. It's the interaction of dozens of tiny parts, like a clockwork skyscraper. Any little failure could be trivial, and affect nothing, or it could be catastrophic, and cause your computer to spontaneously reboot/lockup/lose data.
I'm back on Macrumors recently because my Mac mini 2018 running 10.14.6 had a firmware update rolled into a security update... now, even after a reinstall of the OS, I cannot manually sleep my Mac, it just reboots instead. Before I rolled back, to not having that update, it would reboot if it slept the screen or so with safari open. So I stopped using safari and went back to Firefox, but it still rebooted during energy saver induced sleep... I read that you can't roll back the new firmware type on T2 chips without bricking (breaking) your computer... I've read now too that Apple is aware of this issue and isn't planning to fix it... why? Because their solution is to upgrade to 10.15... oh wait, what about all my 32-bit apps which will not run if I upgrade? I can't get rid of all of them yet!
I just hope you choose the path that hurts YOU the least...
ugh...
Good luck.
