The architect profile does just that.
I can recommend reading the Installation guide - ArchWiki as you likely want to learn how a Arch based system is put together.
Best way to learn is to get an Architect ISO Releases · manjaro-architect/download · GitHub and build from ground up.
As for Manjaro - you may also look at the Packages-Root - technically you can trim that further down - but getting down to 300MiB - I don’t think that is possible.
A few years back when I maintained the openbox based ISOs I experimented on how low I could go.
A stripped down ISO with GUI and basic X support and installer would go around 1.7GiB and consume 100MiB when booted.
You could try this
mv Packages-Live Packages-Live.bak
mv Packages-Desktop Packages-Desktop.bak
mv Packages-Mhwd Packages-Mhwd.bak
mv live-overlay live-overlay.bak
mv desktop-overlay desktop-overlay.bak
Comment content of Packages-Root except for
base
grub
linux-firmware-meta
KERNEL
Edit profile.conf
displaymanager=""
See where that takes you 
I am currently experimenting with wayland-x11 because it has a very small footprint and you can run everything on top of it.
EDIT 2025-10-04T10:07:00Z
The smallest bootable ISO I have been able to build using buildiso and a customized profile is just closely around 500MiB.
Demo ISO at manjaro.dk (cannot be installed - just booted) if you want something installable you need to create a customs script.
EDIT:
ISO profile used