Should MTUs of each single device match the one on WAN?

QUESTION #1:

Title. currently my set MTUs are:

As you see pppoa-wan is 1478 since it should be the best (because how PPPoA protocol do padding).
Should I set 1478 on the other ones as well? O a multiple of that (so internally only one MTU is used but when it comes to go out it splits perfectly? It event works like this or am I saying bs?)

QUESTION #2:

Also, dsl0 IIRC is just a dummy device created by default with the DSL interface mac address placed there (and could be removed with no problem), given that now the pppoa-wan device is a proper entry in the config file, should it be placed there?

Or maybe is like this because the dls0 name is now used somewhere in OpenWRT (or Luci)?

Removed second question. See mbo2o reply.

DSL0 is the physical layer
pppoa is layer2 and passes over dsl0

You need both, dsl0 is not a dummy

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