I have two Openwrt devices, they are connected with wireguard, and inside them there is a gretap tunnel. The gretap interfaces are in the br-lan bridge of both routers. The goal is to be able to create an L2 network on the two routers that are in different places. Unfortunately, routed is not an option at the moment. The gretap interface has an MTU of 1280. In theory, if it receives a larger packet, it should send the system a "Fragmentation Needed" ICMP packet back. However, it does not do this. MSS Clamping does not work, because officially the packets do not pass through the firewall due to the bridges. If I reduce the MTU, that is not a solution either because then the client does not work, because it does not receive the ICMP FN packet back. The WRT discards the packets because they are too large. What did I not enable, or why does the system not fragment the packets automatically? I tried to solve some problems similar to mine that I found on the forum, but I couldn't solve it.
If you have to and want to go the stretched layer 2 way... Booth your LAN need to match the MTU of the gre tunnel without the extra outer frame.
MSS and PMTU do only work on layer 3.
You can patch up tcp init pñckets on the bridge.
you need pre and postrouting towards challenged port. It will not send frag-needed on udp, nor know automatic "rt mtu" ie iif gretap tcp flags syn tcp option maxseg size set 1280
One of the reasons no one runs udp s^slog over jumbo frame
Thanks. That's new to me. And I hope I will never have the need to use that knowledge 
Thank you!
How should I fix this in the system, so that it is always executed when the firewall is restarted? The rules placed in nftables.d were not always included in the system when I tried something like this. Although I tried in traditional chains and tables. Unfortunately, the functionality of the luci firewall is not complete, and even the individual rules that were there in the past have been removed.
I still don't understand something. You wrote " It will not send frag-needed on udp" I wouldn't even distribute it on UDP at the moment. The one that requires L2 fits into the system without a problem. But there are a few devices on the other side of the pipe, especially switches, whose web interfaces are not accessible. These communicate on TCP, not UDP. Sorry, I must have misunderstood something.
You need table-pre or table-post hook in fw4 and your table must create delete and create full table again.