I already bought all dlcs from Expansion Pass 2 separately. Is there a way to get Herald of Glory dlc? Do I have to buy entire Expansion Pass 2 at full price anyway? Any chance for a discount or to make it standalone?

No(t yet). Triumph did make bonuses available separately before for Planetfall, but not for AoW4 so far.

Quote on GOG page: " *This content might be made purchasable individually at a later date "

As we say, "better one now than two later".
So, Paradox tells us we should not buy the DLCs, but wait for the Expansion Pass release...

However, it's in the Collector's Bundle (not in Premium Edition) :
"The price of this bundle automatically lowers based on the games you already own."
so, see if the price fits you

While I do support GOG, on steam you can just complete the pass if you bought the DLCs.

If you can't do that, talk to gog about buying the pass and refunding the DLCs.