UPERFECT Battery Portable Monitor 120Hz Touchscreen, Upgraded 15.6 Inch IPS HDR 1920X1080 FHD USB C Monitor Built-in 10800mAh Battery & Quad Speaker, Eye Care with HDMI USB Type-C Smart Case

The Asus only has 1 USB C port ,so the only way to get it power and video is to use a dock that has USB C output eg Dell Thunderbolt Docking Station WD19S etc. That will charge your phone and charge your monitor. But it is bulky. 1q) does your monitor auto shift to Portrait mode when connected to phone? if no, I would recomend a portable battery monitor. i have this: https://www.ebay.com/itm/314687508918 Plus: Full HD, Touch Screen, 5400mah battery, 1 HDMI, 1 full usb c (for data,video, power), 1 usb c for power, 1 mini USB for otg (mouse /kb dongle etc) works really well on 1 single usb cable when plugged into laptop, phone Can charge the phone wile connected to phone (separate setting) Monitor Menu is touch screen enabled... Phone S21 can power it if brightness is set to minimum. Minus: defaults to charging from phone when battery is less... so my phone kept on connecting disconnecting. Solution is to plug into an external power if the monitor battery is low. on/off status is lil difficult to set, so my monitor sometimes discharge while being in standby ... Magnetic Cover + Stand is ok, monitor is not very stable (slides to horizontal position on slightest disturbance) when using the cover as stand. Biggest: no orientation/gravity sensor... so if used with phone (without dex) there were big wide black bars on either end. If it had a G sensor, rotation monitor to portrait mode would have made the phone screen use the full monitor screen, and the monitor would have become a giant tablet... This can be had around $150 if you search on ebay.

Can also vouch for the UPerfect USteam E6 Pro touchscreen gaming monitor (18.5"). I'm using as a monitor for an exercise bike with video provided via DEX using a s20FE and it is working great. Very low cost. For a USB-C monitor (also has HDMI in). https://www.uperfectmonitor.com/products/1920x1080-monitor Video review here and a coupon code noted in the video's description for additional money off: https://youtu.be/zWCIZ62zq0w?si=iwjC82-3B7tsZKBE

I have one of the UPERFECT battery powered portable monitors, works quite well. Surprisingly good battery life too, assuming you’re not cranking the brightness. Significantly better than the Ally at least