Due to my incompetence in CZ I need to respond in english (and I only got the gist of your posting from google translate):
"Hi, I have a problem with the fact that sometimes a device loads inet. so much that practically nothing can be done, even DNS queries will not pass many times. This is mainly done when backing up photos from your mobile to the cloud, uploading a larger file to a Google drive, etc. My upload is weaker and, as it gets overwhelmed, other devices do not get stuck.
Fortunately, the problematic devices are on a separate WLAN, so I set the wlan via LuCi upload / download limit so that it can not get the full line and everything works beautifully, BUT … then it was necessary to work with a local NAS and of course it was useless and I had to turn it off.
Is there any way to set the traffic to the LAN from the WLAN to be unrestricted and the traffic to the Internet set speed limits?"
So what I would recommend is to simply use SQM on the wan interface and configure per-host-fairness as described in https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm-details in the section titled " Making cake sing and dance, on a tight rope without a safety net (aka advanced features)".
If successful this will fairly distribute download and upload bandwidth equally between all concurrently active devices, which should solve your 'blocking" issue, while giving uploads all bandwidth if nothing else wants to use the uplink.
P.S.: it was the SQM keyword that “triggered” my response…