In a Macbook, I'm ready to change the boot disk and replace the superdrive with an SSD. How will Migration Asst work? Use APFS? In a mid-2012 MacBook running Catalina, I'm first going to replace the DVD SuperDrive with an SSD. Boot-up and then make sure the drive is recognized and wifi, Bluetooth, camera, etc. still work. ok.
Next, I want to replace the current drive with an SSD. However, I've looked over Migration Assistant and I don't understand my best option. Here is situation:

each of the new internal drives are 1-tb Samsung 860 EVO and will be formatted APFS.
the current drive is a 250-gb Samsung 840 EVO with 80-gb free space and is formatted APFS.
a TmeMachine back-up exists on an external usb-drive formatted HFS+.

note: I have an external monitor connected via USB. The Catalina upgrade required some difficult security configuration to get it to work again and I don't want to redo this. I've also got a mysql database that I don't want to reconfigure with this upgrade.

Do I need to do a fresh install of Catalina? And thus have to re-configure my usb-monitor?

Can I mirror the current 250-gb drive onto the 1-tb SSD that replaces the DVD superdrive, and then mirror back that image onto the second 1-tb drive that will replace the 250-gb drive? My usb-monitor and mysql database shouldn't need re-configuration.

Will APFS not supporting TimeMachine effect this migration?

Or, just what is my best migration option? My current MacOS, app, and data arrangement are just like I want. Now, I just want to replace the disk it is on and add an additional disk. I've got plenty of cloud and extra usb drive storage to work with.
Personally, I've lost all faith in Time Machine in recent times.
I'd approach this as a cloning exercise.
I'd go with Carbon Copy Cloner [just because that's the one I know best] &amp; do as your 2nd suggestion.
Clone to the DVD replacement, test, add the new larger SSD in place of your old HD, clone again. Remove [or just temporarily disconnect] the 1st SSD to test again.
If all is successful, wipe the intermediate SSD. If at any point something goes wrong [which barring human error it shouldn't] - you still have the original HD as belt &amp; braces.
Once finished, neither you, your software, or your sql will know the difference.
One final check once you're done - open System Preferences &gt; Startup Disk &amp; make sure the new boot drive is already selected [it should be, but I've known it miss very rarely]
One thing to note when cloning:
Rename the new drive to something else first, so you will then be cloning MyDrive to MyDrive Clone.
Once the clone is complete &amp; you are sure you can boot to it, change the names again, to MyDrive [from MyDrive Clone] &amp; MyDrive Old [from the original]
This
a) makes sure you don't lose track… or indeed your sanity &amp;
b) keeps anything that was reliant on the drive name sane too.