MTU wireguard + VxLAN + batman-adv

I have a lets say complicated mesh setuop.

I have a full batman-adv wireless mesh over 802.11s that works as expected

However I have for some routers an ethernet link that I can use as backbone for nodes. These nodes are running over ethernet : wireguard +VxLAN (to get L2 as batman-adv) and on top of that batman-adv (as it is very good on loop detection and automatic mesh forming in case of failure) .

Now everytime I do

ip link add dev vxlan master bat0 - i get the message : The MTU of interfac vxlan is too small to handle batman-adv packages. Pakets will be fragmented ...

And then the bat0 interface set itself to 1500 MTU even I have set it previousy to 1310 MTU.

What is the best practice in setting up this combination ? and actually how can I see if packets are fragmented or not . In fact I could calculate myself the values but how can I test paket fragmentation ?

The devices are so underpowered (single core) and the fragementation impact big enough.

Now I have the follwoing MTU's for eth

eth 1500
wg0 1420
vxlan 1370
bat0 1310 ( I also have a vlan bat0.10)

how can i test these values ?