[Jari Innanen]

You can put the letterbox mask on an adjustment layer.

[Jari Innanen]

It is EBU R128 in Europe:

https://tech.ebu.ch/loudness

[Jari Innanen]

Select the clips in the Browser and set the Spatial Conform in the Inspector.

[Jari Innanen]

Make a 1080 Project. Set Spatial Conform to “None” on 4K clips. reframe as needed. Export.

[Jari Innanen]

Compound inside compound spells trouble. I hardly ever use compound clips. So don’t if not necessary.

[Jari Innanen]

Check this thread:

https://forums.creativecow.net/readpost/344/29764

[Jari Innanen]

With Space Designer you can achieve good neighbor effects. Good info on FCP X Logic effects here:

https://help.apple.com/finalcutpro-logiceffects/mac/4.0/en/finalcutprox/logiceffectsreference/Final%20Cut%20Pro%20X%20Logic%20Effects%20Reference%20(en).pdf

[Jari Innanen]

You may need to press the Option key while you click Scaled to see additional resolutions for the HDMI monitor.
https://support.apple.com/kb/PH12783?viewlocale=en_US&locale=en_US

P.S. I see now you have an Ultrawide display. I have one also, but haven’t tried to use that one as an fullscreen output. Only as the main monitor for the timeline.

[Jari Innanen]

Also the reso should be set to 1080 on Asus.

[Jari Innanen]

There is also a setting in the Import window, where you can hide already imported clips.