Is Ubuntu-Mate the right OS for streaming with ffmpeg on a RPi4b?

I need to create a stand alone streaming device to take the h264 IP camera output, decode it so that I can change the image window ( zoom in 50% ), encode it back to h264 while adding an external audio stream and feed it to Facebook in real time. I need an OS that offers the minimum CPU overhead and is geared to taking advantage of hardware decoding using ffmpeg. I tried the Rock Pi4 B 2GB with it's claim of high speed and was very disappointed. It performed in this function, sadly, not any better than a RPi3B+. I was also disappointed by their lack of quality. I couldn't get any of the operating systems to fully function with alsa and the pulseaudio services. Also, their uSD card slot was very shallow and sloppy. It's easy to destroy the uSD card while using their device. I just ordered a Raspberry Pi4 B 2GB unit and I will try again.

Ultimately, how would you-all think about Ubuntu-Mate measuring up to the other 64 bit OSs for this purpose? Also reading up on the pros and cons of 32 and 64 bit OSs, for video manipulation, would you think a 32 or 64 bit version be better for this?

Currently, there is not a version of Ubuntu MATE for the Raspberry Pi 4, though we have been assured it's in the works. With 2GB of RAM, there won't be a huge (or maybe even noticeable) performance difference between 32 bit and 64 bit.

Will the 32 bit Mate run in the RPi4?

I do not believe so, due to the fact that the firmware is different. However, some people have reported success by basically copying the firmware from Raspbian to Ubuntu MATE: How to Get Ubuntu-MATE 18.04 up on Raspberry Pi 4

I think you will not get a favorable rate when doing this with an underpowered device like a Pi. But if the rate is acceptable to you, I would go headless, because if this is all happening automated in real time, a GUI might just take memory and processing power for no benefits to the task at hand.

Thanks for your input. I agree with you 110 percent about the limitations of a device like the Pi. I have been doing a lot of experimenting with the Pi 3B+ over the past month and ffmpeg streaming. You would be surprised that this device can actually handle bringing in an rtsp h264 stream and push it out it to an rtmps site without difficulty. The problem is when you try to decode the stream and re-encode it after a minimal amount of processing, that's where it falls apart.As a result of this, I gave up on my project.

Since the introduction of the Pi4, I gained new hope. I ordered, not the Raspberry Pi4, but the Rock Pi4B out of China for it's superior specifications. Boy was that a mistake.
I messed with that device for over a week until I bumped the uSD card and busted the flimsy, sloppy, shallow card slot they use. Due to all of the problems trying to get the audio set up in any one of a bunch of it's OSs, at that point I just gave up on any hope of using the Rock. Have you ever tried to get support out of a Chinese based company?
That's when I ordered my RPi 4B/2GB unit.

Right now, after making this post here, I discovered that there was a build of Ubuntu-server 64bit available. I downloaded it and installed it. Here again, I am now wrestling with trying to get the audio set up in it with out any luck. I then went back to the latest Raspbian release thinking that if I can get it working in Raspberry's OS I could port those setups to Ubuntu-server.

Right now, I am using Raspbian as a playground. My goal is to get my project running headless on the RPi4 with Ubuntu-server.