I see a couple of issues here.

First. the length of the throttle horn is important. To get decent "thumb to throttle" response, you are far better off to use 100% of the servo travel for the throttle. This is the reason for the extra arm length. Use the additional arm.

The snap on ball joints are not a good idea for the throttle linkage. There is a high likelihood of failure. Use either a bolt though ball joint or a clevis. If using a clevis, use it on a plastic arm but expect the hole to wear fairly quickly. (My recommendation is to use a bolt through ball joint).

If you can not obtain some rod that is threaded at each end, Can you get a soldered on rod end? Some pull-pull connectors can be used for this as they have a hollow in the end. z-bends and easy connectors tend to get a lot of wear fairly quickly on gasser linkages.

A long metal rod going from the engine to near the receiver can be an issue for radio interference. Have you asked any of your flying buddies if they have a spare nyrod you can borrow until it becomes available?