Asus AiMesh (RT88 & RT86)
Ah, you have lots of router experience I see.
I have tested out lots of brands. And 100% agree with the comment about Netgear firmware.
This is a bit of ramble because I doubt we are going to shine a lot more light on what you already know.
I have used Asus models you had and my current main router is RT-AC88U on Merlin. I used an older 68U to test the mesh and overall can say it worked but unimpressed even with ethernet backhaul. In fact the Asus worked better in AP mode which does need ethernet.
As commented.. I don't think any mesh system works great if it is an add on to regular wireless router. It really needs to be built ground up as mesh product. And to use separate wireless backhaul rather than trying to mix connectivity and backhaul into one wireless chip which causes massive slowdown.
Asus have released AX models for mesh. The products are pretty raw and although bug fixes will happen in due time and Asus is great in keeping their firmware updated, I would not jump onto the bleeding edge unless you are prepared to bleed out.
I do think your best bet is keeping the Synology as main router. If it still has 5ghz problems when not running mesh perhaps Synology can replace it. 2600 is both fairly old now and is expensive unit and should have decent warranty. But the router features are excellent (much like the NAS).
Then Netgear Orbi but purely as wireless AP. At some point ethernet can be installed for backhaul.. and that will certainly improve things.
I did also setup Ubiquiti with Unifi UAP. It is a great system. The learning curve is pretty steep. But having got over that, I hit a roadblock in USG router having issues with my ISP. I was forced to rethink the whole thing because I did not want to use command line configurations. I lost a bit of money but in the end, realised it was beyond my needs and if I wasn't around nobody else could ever understand how to fix it.
That has made me rethink also using simpler AP units so it was not necessary to get a controller.
I found TP-Link has a range of pro APs. When I say pro I mean better than domestic.. EAP225 and EAP245. They are fairly cheap.. and although it looks like it needs ceiling mount they can be easily mounted in other places. The have a full gui onboard. You don't need a TP-Link router or anything else to use them.. and they are better than most. Certainly the signal / coverage is excellent. What you miss is ethernet ports.. which is also why I prefer the Netgear Orbi higher models RBK40 or 50 since running AP at desk level you may as well have the switch combined. A few ports makes it a lot more useful.
I do find Netgear a bit behind in Apple compatibility. Have been working through issues with M1 and M2 LTE routers and a lot of issues seem to be Netgear poor handling of bonjour.
Ethernet though.. at least it is dependable, reliable, fast and you can use cheap as chips wireless routers as AP.
End ramble