I've been using it for months with no issues (I did my own build via the branch on the development thread) but now it's been mainstreamed so the latest snapshots as of today have it. No need to roll your own anymore.
The only wrinkle is that they don't include the DSL firmware file itself (licensing issues) so there's no DSL access from the start, you need to import the DSL firmware file into the box after you first install OpenWRT.
Snapshots:
https://downloads.openwrt.org/snapshots/targets/ipq4...
How to get firmware file:
https://github.com/openwrt/openwrt/commit/539b52068a...
Wiki (not yet updated but will be soon):
https://openwrt.org/toh/avm/avm_fritz_box_7530
Forum:
https://forum.openwrt.org/t/adding-support-for-vrx51...
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