what is best way to clone external SSD as startup drive?

What you want to do is: when you get the external, plug it in and first erase and format it Mac OS Extended and GUID partition scheme using Disk utility. After that, fire up CCC or SD and simply tell it to clone to the SSD. When it is finished (will take a while for a fresh clone), test it by booting into it to make sure everything works as it should. Then you can either unmount/unplug it or leave it to use as your main drive. You can then use the internal as a backup or additional storage (in that case, wipe it first).

Use either SuperDuper! Or Carbon Copy Cloner. Both are great and do essentially the same thing. Both have trial versions however I’d buy it and use as another method to backup in addition to Time Machine, when it comes to backing up having redundant methods is smart!

Yes, those are all good methods. But in addition to performing a one-time copy of an entire drive like Disc Utility.app, the SuperDuper and CarbonCopyCloner software offer some very useful features you may consider for making regularly scheduled backups, which efficiently only copies changed files since the last backup was made.

/Applications/Utilities/Disc Utility.app can be used to copy an entire drive:

http://www.stellarclonedrive.com/disk-utility-to-clone-mac-drive.php

that is an older version which looks a little differently, but you use the same "Restore" option on the destination drive.

Good question, I am sure others searching the forums with this same question will find this useful in the future. Good luck.

Some other options:

https://www.retrospect.com/

At one time, DejaVu came bundled with the disc burning software Toast, https://propagandaprod.com/

Most of these are actually running a UNIX utility, with the Mac-aware options, in the background, if you are familiar with the Terminal.app you can also explore the equivalent command lines to do the same thing, but without the fancy GUI. Not recommended for non-expert users.

CCC, it will also give you the option of cloning the Restore partition.

I second using Carbon Copy Cloner.

If you are using the external as your "working" drive with the OS and all apps, then your internal drive is no longer needed - unless you want to now use it as an extra backup. You could clone back your new external to the internal to create another bootable disk. Or you can erase and use it as a drag 'n drop backup disk or you can use it as additional storage if you need that (i.e. keep your Photos library there to free up space on the external depending on its size). And yes, anything you use it for will be slower, but for a backup it isn't that important. Any "work" should be done on the external - that is simply your new "hard drive" that you are working with.

And yes, any downloads, purchases, whatever should go on the external - should be rather easy: you are booted into it anyway.

Ok, normally the OS will automatically convert to APFS if needed - and I do not use SD, so I am not sure. And, all my externals are Mac OS Extended. But, if it is an SSD and required, I'd probably use the plain version. Since I am not sure, wait for someone else to confirm (or not).

You're welcome!

You are welcome and good luck!