I can't find a matching profile. What do I need to do to get a current version of OpenWRT on the device and bundle several packages in the image to save space.
I think it is to bad to put the old hardware to trash just because I didn't dare to compile a current firmware. It should be possible to get a current version running on the device, right?
At 16 MiB flash and 64 MiB RAM, you should still be able to use it (but a web UI might be a close call). What first needs to be done is port it to ath79 (which is the device tree based equivalent of ar71xx).
Since ar71xx is deprecated, all hardware that was not explicitly ported when migrating to ath79 has been dropped. It needs to be done on a per device basis.
Yes, this will work. You will also need to create an image profile (see target/linux/ath79/image/generic.mk).
As @Borromini suggests, you will need to be able to recover out-of-band. For example, you should be able to boot an initramfs using your serial console to control U-Boot.