The procedure you used to manually install the current Windows 10 (21H2) appears to be correct. You state to be having problems installing Windows updates. While I assume your question is asking what you are do wrong, you should consider the following.
From everyman.com, below is a list of all 2015 Mac models. Note that only the Mid-2015 MacBook Pro models are using 4th generation Intel Core i7 processors.
Apple states all 2015 Macs are Windows 10 compatible. However, Microsoft lists all processors used in 2015 Macs as Windows 10 (21H2) compatible, except for the processors used in the Mid-2015 MacBook Pro models. In fact, none processors used in the Mid-2015 MacBook Pro models are list by Microsoft as compatible with Windows 7, Windows 8.1 and all releases of Windows 10.
So you may have to try different releases of Windows 10 and/or restrict a Windows installation from installing updates that may be causing your problem. Evidently, Rufus can be used to download older releases of Windows 10.
Using Rufus to Download an Older Release of Windows 10
From Windows, download the portable version of Rufus. I use a VirtualBox Windows 10 virtual machine on a macOS host.
Open Rufus. (In my case, this was
rufus-3.18p.exe.) If there is no down arrow next to the
SELECT button as shown below, then click on the "Show application setting" button and change "Check for updates" to
Next, close and reopen Rufus. There should now be a down arrow next to the
SELECT button, as shown below.
Click on the down arrow and select
DOWNLOAD. The result is shown below.
Click on the
DOWNLOAD button and wait for the popup shown below.
Select the Version, Release, Edition, Language, and Architecture, as shown in the example below.
Click on the Download button, then select where to save the download. The download should proceed, as shown below.