Difficult situation, in particular in connection with these abroad shipping situation. I do not really know which law is applicable, a science for jurists.

Basically according to German law in case of costumer complaint ("Nacherfüllung") the seller is responsible for the shipping costs of the return.
But you can not simply insist on getting a new item shipped, without having the damaged one returned!

My procedure in theses situations is to take the way which is intended by law. I.e. I would return the wheel insured to bike_discount. And I would claim the returning costs from bike-discount. In a last attempt I would sent an E-Mail (do not leave bike-discount an option, but just explain how you will proceed, why (reference to law) and what you claim) and that you expect your expanses returned. Mention that you are prepared to take legal actions and intend to give negative evaluations (?) in corresponding online platforms.

According to German law (you get an overview in German here, the relevant article is § 439 Abs. 2 BGB, BGB is "Bürgerliches Gesetz Buch", the basic civil law in Germany) bike-discount has to take all the returning costs. It is important, that you refer to Nacherfüllung, formerly also called Reklamation, but not to Widerruf (which is just the buyer-option "I don't want it (for no reason to be specified)"). In the latter case you would be responsible for the returning costs (§ 357 Abs. 6 BGB). It is your task to select the carrier, choosing a tracked shipment is a reasonable aspect in this context.

If there is no return for your e-mail, just sent also a physical mail with the same content and don't forget to specify a deadline until when you want the shipping costs returned and that you want either the money for the wheel back, or a fixed one returned (I would keep the decision free to bike-discount).

P.S.: I thought that DHL-shipping of such large items like wheels is always tracked and insured (up to about 500€), isn't it?
And take care that you secure the dynamo axis more properly than it had been done by bike-discount!