Sonos Arc Ultra era 300s and 2 Subs 4s not loud enough

To have a common level to compare; have you tried to play music directly from the Sonos app? In this case you will have the source directly from internet to the speakers. 

With a lighter set-up (Arc+Sub3+2Era300) I never go over 50% of the volume. I suppose that during a party with the room full of people could reach 100% before the police arrive 😅 but strong enough antway.

I would check deeply the Apple TV or the display settings. Or perhaps you have a living-room like a church, with ceilimgs over 10 m and 200 m2 😉

Yes, I've used the Sounds app's streaming service, and the volume results are the same.
I want to clarify that for some videos, the sound is acceptably loud. When used with some videos or music at volume 70, I already get good results (which, in my opinion, should have the same result at 50%).
The problem is that with certain sources, as I mentioned earlier (Superman 2025), I have to raise the volume to 90% to get results that are far from what I expected for that volume level.
My living room is approximately 40m2.
Having used a BOSE 600 with a 5" BOSE subwoofer in the same room and getting practically the same volume results, I'm actually considering replacing the entire system. It's a real shame because the sound quality, especially Atmos, is sensational. Sonos did an excellent job delivering sound quality.
I might try the Sony Bravia Quad Theater. I'll test it before I buy.

You might want to consider a component setup with a high-power multi-channel AVR powering individual speakers. Not as convenient as an integrated system but far more volume potential.

You`re right. Since I love convenience, I'm going to try to “test drive” some soundbar systems.
It would be amazing if Sonos released more power for this system in an update. I don't know if that would be possible. If amps are already almost at the limit.

Since my Arc Ultra, Sub 4 and Era-100 surrounds will play far to loud for comfort in my very large room I don't think it is a Sonos amplifier issue, unless yours is broken.

Stanley, you make me jealous LOL.
Now my system is drastically lowering the sound alone at times. I really must have a problem with the system.

I was having problems with momentary sound cutouts at certain times. I discovered that with the microphone switch on, Arc Ultra understood that I wanted to "say something to it" and lowered the volume momentarily. After turning the microphone off, it never happened again. This might help someone with the same problem.

Apparently, with the latest update, my system no longer has any delay in releasing sound when it's turned on. What used to take 3 to 5 seconds to hear any sound is now instantaneous. I don't know if this happened due to the Sonos update or if my TCL had some kind of update that caused this.

I discovered what fixed the delay. The latest Apple TV update brought a feature called Continuous Audio Connection. That's what solves the problem; however, the Sonos app will display any content as Dolby Atmos, even if it isn't (YouTube, for example).

That’s unfortunately a reporting issue by Apple. Sonos displays what it is handed.