First of all, yes - you should update, if there is no ath79 image, your only option will be ar71xx.
No, that would (even if the router would accept the wrong image) brick the device - as the devices are not the same (the different switch needs its own configuration, force-flashing the wrong image would end up without any ethernet support).
It's not really strange at all, porting device support requires someone with the device on their desk to actually work on it - and the tl-wdr3500 was a much less attractive (and therefore less common) device than the tl-wdr3600 for just about the same retail price (about a fiver difference). Buying the tl-wdr3500 instead of the tl-wdr3600 has never really been worth it, unless it was bought in quantities (>5) and only intended to be used as AP (with no focus on wired routing).
20.xx.0 won't ship with ar71xx anymore (the target is pending removal), so unless someone (you?!) does the necessary porting work, the tl-wdr3500 will cease to be supported. The porting work shouldn't be too difficult (because of the existing support for the tl-wdr3600), but might still face some issues because of the different (uncommon) switch setup.