I think you are missing the point here.
The point is - the system is one big intervowen mass depending on eachother.
While it may work - it also may not work - and if you are getting weird issues because of it - you are on your own. Also know that discussing issues and solutions requires a fully synced system - anything else is unsupported.
As it is your system and you decide what to do and how to maintain you should just install the package using
pacman -S pkgname
You can use lftp to create a copy of the Manjaro branch you are using - then setup a caddy webserver to serve the files. It doesn’t have to be complicated - in fact it can be your own computer - suffice theres is enough space.
Install the packages lftp and caddy
sudo pacman -S lftp caddy
Then copy the branch from your preferred mirror e.g. stable to your home (58G)
lftp -c mirror https://some.mirror/manjaro/stable /home/$USER/
When the copy is done start caddy
caddy file-server --root "/home/$USER/stable" --listen :8080 --browse
Edit your mirrorlist /etc/pacman.d/mirrorlist and insert a line at the top of the list
Server = http://localhost:8080/$repo/$arch
Disable the pamac-mirrorlist.timer
sudo systemctl disable --now pamac-mirrorlist.timer
If you ensure the caddy server is launched at login you can control when the repo is synced and you can install packages without syncing as you know the state of your system and you know which packages is in the repo.
When you decide it is update time you run the lftp command and refresh your copy of the repo. I know there is a switch for the mirror command to only fetch changed files. After you refreshed the copy you run pacman -Syu and your system willl update.