I may be a little late answering.
Installing windows first is easier. I say this because the windows installer is quite finicky. It wants certain partitions of certain sizes in a certain order. (varies across windows versions and even custom ISOs). You can use
gparted on the manjaro live usb to change these partitions. Installing manjaro second will make installing grub much easier.
If you install manjaro first, be warned. The windows install will overwrite your grub loader. You will need to
chroot into your manjaro build and fix it.
If you have two physical hard drives (not partitions), life is a little easier. You can create an efi partition on each drive so that windows can have its own. The grub installer will detect windows and add an entry for it. In your BIOS you can choose which UEFI to boot first (grub or windows). You may need to install windows on the first physical drive for this to work. When I tried installing windows 1903 yesterday I had several problems, though I switched to a custom ISO to force it to work.
Once everything is installed, configure manjaro to use the RTC time. Windows sets your BIOS time to local time (whatever time is in your timezone). Linux sets it to use UTC time (the time in London).
If you have a 5700/XT graphics card installed. You may want to use on of the manjaro development ISOs. There is a possibility that you will have no GUI and have to install the necessary drivers from the command line. It would be easier to install manjaro with your old graphics card, install the drivers THEN switch to the new card.