Unfortunately this isn't something you can change directly in the UI config today since the sort order for that lookup is controlled by the platform, not a setting you can toggle.
A couple of workarounds in the meantime:
The search/type ahead in the topic field does work, so training users to just start typing the topic name rather than scrolling the full list is the quickest short term fix.
If you have access to customizations, you can look at whether the topic lookup can be replaced with a custom control that applies a sorted view, though that's more involved.
The best path forward is to submit this as a product feedback/idea through the Dynamics 365 Ideas portal at aka.ms/ideas. This is exactly the kind of UX improvement that gets picked up for release waves when it gets enough votes. Given the screenshots clearly show the problem and the use case is straightforward, it has a good chance of getting traction. Make sure to include the before/after comparison just like you have here as it makes the case very clearly.
