Adding multiple IPv6 prefix support in LEDE?

I know LEDE is more or less outdated (though it received some security fixes in the last weeks, jumping to version 17.01.7). However, I have a unit that won't work well enough with 18.06. Thus, I'd like to stay on LEDE.

However, Luci in LEDE doesn't work with multiple IPv6 prefixes. At least it's possible to add them manually in /etc/config/network (space separated list). I need this for the multiple prefixes from my HE 6in4 tunnel.

Now there have been some commits that supposedly fix this issue. So I tried to add them manually into my LEDE source (based on the most recent LEDE sources), as depicted here:
which is referring to these commits as fix:

So, after that, Luci is fine and not complaining. But the routing for the prefixes doesn't work. Prefixes and addresses get assigned fine. But even though it's in the /etc/config/network, I have to add the route manually.

Actually, I'd be fine just fixing Luci. But I don't know how I should parse the change of
-ip6prefix.datatype = "ip6addr"
+ip6prefix.datatype = "list(ip6addr)"
So, since datatype is now "list", it's stored in an array, I guess? Has anyone tried 18.06 with multiple prefixes and knows if the issue is really fixed?