Alright, I think I finally figured it out.
I used to follow the steps below to produce a custom symbol when using 2.0 templates:
Export similar 2.0 template from SF Symbols
Import template in Adobe XD
Edit/create custom symbols in desired scales and weights
Export as .svg
Import in Xcode
Now, it seems that you have to import it back into SF Symbols in-between 4 and 5, you can't bring it directly into Xcode. The steps thus become:
Export similar 3.0 template from SF Symbols
Import template in Adobe XD
Edit/create custom symbols in desired scales and weights
Export as .svg
Import into SF Symbols
(Set up layers and colors for the various rendering modes)
Export symbol
Import in Xcode
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