I think you either misread or are a bit confused about the process.
You cannot use a NOOBS to setup a Rpi to boot from an SSD via a USB adapter.
Step -1 Just start from scratch. Format your SD card and your SSD. Download the pure Rpi OS IMG files from here. And then flash the same Rpi OS IMG file twice, once to your SD card, and once to your SSD - using one ...
You obviously ignored the Foundation advice to perform a fresh install.
It is futile "cleaning" the boot partition because every time you try to upgrade the firmware the files will be restored.
Your only option is to adjust the partition sizes (which can't be done on the Pi).
You will need a Linux computer.
There are numerous other hidden problems ...
Burried in the Buster release notes for the version 2019-06-20 is a small entry that states:
Boot partition size set to 256M
This is to allow for larger software upgrades (shades of Windows 10) and more software (e.g. Pi 4 firmware) to be shipped as defaults on the FAT32 portion of the SD card.
If you do not want to reload your card from scratch then I ...
The ideal way to do this is to expand the filesystem, which requires moving the second partition.
However, if you don't want to bother with that, a lot of the space is taken up by the biggest files, which are the kernels and start_elfs. If this card is only used in one Pi, you don't need all of those. If it is a single core model, you only need kernel.img, ...