Is the remote the only difference? I recently replaced a 2017 SS+ with a 2019 SS+ and am now noticing for the first time that the audio is out of sync on the Amazon Prime video app. I've done all the troubleshooting, made sure volume mode is off, removed restarted and re-added the app and no joy.
I no longer have the 2017 SS+, so I can't verify if audio was in sync or not, but I do know it is perfectly in sync on my ordinary Streaming sticks on other TVs, and on the same TV when using Chromecast 2nd Gen (casting from iPhone Prime app) and on AppleTV 3rd gen. It seems the Prime app has audio sync issues on my setup only on the 2019 SS+.
I'll consider seeking out a 2017 SS+ if I knew for sure that would solve the problem, but if you're saying the only difference is the remote, then I'd be better served by something else.
Update: disabling the advanced system settings - advanced display settings feature "auto adjust display refresh rate" appears to have solved my Prime audio sync issue.
However, don't forget the RokuOS and/or Prime app updates that most likely occurred since you had your 2017 SS+: firmware 9.3.0 first rolled out in April, and has had at least 5 different build releases (depending on model/release group), and there have likely been as many Prime app updates during that time too, either/both of which could be/likely is central to the issue. So unless your old SS+ is at a pre-April firmware/app version level (for comparison testing), its mostly moot.
But yes, that's the only difference (aside from some interesting initial Theme UI differences for the 2019 models that disappeared by the time of the RokuOS 9.3.0 rollout) - they have the same internal/external model identifier (50). And you can always buy a new enhanced remote and pair it with the 2017 SS+ to get the same remote functionality (mute etc), aside from hard-programmed channel/app buttons.
This occurred on a 2017 Streaming Stick+ as well. Another workaround was to skip forward or back, after that the sync was OK. The problem also occurred with some videos in the BBC iPlayer streaming channel in the UK. The problem was fixed with a Roku software update.