One more difference: ath79 support for WDR3600 has been added on 22. May 2018, well before 19.07 has been branched.
ath79 support for WDR3500 however has only been added on 03. September 2019, which is after 19.07 has been branched.
That's the reason why you don't find WDR3500 in ath79 in 19.07.0.
The reason why you won't find it in 19.07 has been given above already by slh: Backports seldom happen.
If your initial question (why there is no stable image for WDR3500) has been answered, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.
That question would be worth a separate new topic.