Fritz!Box7430 capped at 22Mb/s

Yes, you can extract firmware binaries from that file. Instead of unsquashfs4-avm-be from freetz-ng you need to use the *-le version, or just use upstream unsquashfs instead.

It is also possible to use firmware files from other devices, for example version 8.D. from Speedport Pro works well here (extracted using binwalk).

However, I would avoid replacing the aca_fw.bin and ppe_fw.bin firmware files. If I remember correctly, the versions included in AVM's firmware are actually older than the ones from the prpl repository. And the ACA/PPE firmware from other devices might not work at all, because there are different versions of these for different SoCs (in the worst case, the device might crash during boot, so you need to recover using OpenWrt's failsafe mode or do a reinstall).

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