Am I sure ? Good question with Draytek WiFi from what I read perhaps. Well so far the difference is quite dramatic. I have several IOT devices that, as they often do, only run on 2.4GHz and I run my IOT network on a separate VLAN like many of us. With the interfering channels (my bad) - several of the IOT devices were very unreliable - probably because the AP903 (on the same channel) is fairly close to the 2862n. For the last 24 hrs 2.4GHZ on the 2862n has been rock solid but never say never - as you say I should wait a while before relaxing about this 😉 – I’ll keep posting here. I spent a ton of time troubleshooting without success because, incorrectly, I thought I had already separated the channels, which of course is the first thing you should check.
In my config Band Steering is on, and minimum RSSI is off, on the 2862n and 2 x AP903s. One thing I did do was put 2 x 5dBi Alfa antennas I have on the 2862n – makes no difference !

I tried all this, including external antennas, as did others. Pursue it and hopefully it works for you. I even went as far as buying a new model Draytek (3 in total). Still the same issues. Eventually, for me, there was only one thing left to try. Another brand, for wifi at least.

No solution is perfect, but there are signs for me that Draytek are not putting enough resource into their hardware, development or support for me. For instance, why isn't the GUI mobile friendly? Why can't we easily see what processor/memory are in each router before we buy (at least I couldn't)? These are things many other brands openly do.

The one thing I will say is that their response to security incidents has been pretty good. And their basic router reliability seems good (only one hardware failure so far).

Looking closely at Ubiquiti routers though - imagine being able to see in one graphic, without any reconfiguring/extra software, how much of your bandwidth is down to streaming/games/social networks etc. And a constant hardware display showing current upload/download speed. Not tried it myself, but seems like all the things I'd want. At the moment my Draytek is holding up, but we'll see.

Totally agree - had a good look myself & the UniFi software looks to be special.
Put even more separation onto my 1 x 2862n, 2 x AP903s so now no overlap on 2.4Ghz, & I set the channel bandwidth to 20MHz - it's only IOT devices after all.
Now I get 2.4GHz reliably upstairs from my 2862n, I couldn't before, but honestly I could throw a paper areoplane out of my upstairs rooms, down the open plan staircase, & get it to land on the 2862n, so this shouldn't be hard !

Before I kick off, in case Amazon & TP-Link folk are watching, the below is just what I suspect/imagine, & may be completely wrong.

One thing I’ve found is that Amazon (Alexa) smart plugs store your SSID & password in the cloud (“…in an encrypted file”). The Amazon (Alexa) app asks for permission to do this. They do this to make it convenient for you to subsequently add new smart plugs to your WiFi network. You can delete all the WiFi passwords Amazon saves & start over - Goolge "Saving Your Wi-Fi Passwords to Amazon FAQs" & you'll get the page.
I have, or rather had, some Kasa KP105s plugs. I think Kasa do something similar to Amazon but rather store the SSID password on your smart phone. This is only a theory because deleting the Kasa app & “supposedly” all it’s data, & then re-installing the Kasa app & reconnecting a deleted/hard reset KP105 smart plug, the Kasa app will offer a choice of “previously used” SSID passwords (so not all your Kasa app data got deleted when you deleted the app ?). Whereas installing the Kasa app on a smartphone, which had never previously had the Kasa app installed, & then signing into this Kasa app with my Kasa account, it asks you to enter the SSID password again, which presumably it wouldn’t need to if it had stored it in the cloud as Amazon does.

Apologies if I am telling the Draytek community something you all know.

I remember trying to get my head around all that too. I would have devices falling off the network all the time. And the Draytek Syslog tool didn't even log much to do with wifi clients at all. With one Unifi device, I created a mixed wifi network, and two individual bands in 2 minutes, and switched off the Draytek wifi. Everything then connects without me having to go through the troubles we both have with Draytek. Graphical view of all connected clients, at what speed/quality, and what data its using.

I'm networking anything from off the shelf IOT devices (some the same as yours) to individual esp32 wifi chips. All with varying wifi needs and distances. Relunctant to say goodbye to Draytek wifi, but I was on this forum for 3 years trying to get a solution. I just saw more people asking the same question with the same wifi issues. No firmware ever fixed the issue, no response from Draytek, and yet I foolishly still wait in hope. Lets hope you continue to have good luck.

I have edited the original post above in which I had suggested both the BSSID and SSID plus SSID password was being stored/used by Amazon – causing smart plugs not to roam – that’s wrong.
For a test I deleted all my SSID passwords from Amazon, connected a hard reset Amazon plug to a new 2.4GHz SSID on my 2862n, created a new second 2.4GHz SSID on an AP903 (same name and password as the new SSID on the 2862n), closed the 2862n SSID, and the Amazon plug roamed/connected to the second (new) one on the AP903 – clearly by just by knowing the SSID password. I had a lot of problems with Amazon and Kasa 2.4GHz smart plugs dropping off my 2862n and connecting and sticking to my AP903s with poor RSSI – I assumed they could do this because passwords and BSSIDs for the AP903 SSIDs were known to Amazon and Kasa because I had previously connected their plugs to the AP903s with their apps (I have plugs in two different parts of my house) - but this test proves me wrong (for Amazon at least). For some reason, in my config at least, smart plugs seem to “prefer” the AP903 SSID as opposed to the same SSID/password on a 2862n, even though it connects with a poor RSSI. So rather than suggesting, as I had, that smart plugs don’t roam, they rather roam a bit to freely. I have no idea why smart plugs seem to “prefer” the AP903 SSID in my house – bad SNR (signal-to-noise ratio) stuff happening near my 2862n maybe ? I know you can block a client on the AP903 SSIDs but that just made things even worse - I suspect because the client just continually kept trying to get onto the AP903 and getting contunally disconnected. The workaround is simply to have unique 2.4GHz SSIDs for all IOT devices you want to connect to each AP but that shouldn’t be necessary.

I ordered a Unifi AP after this test ;)

My 6 Kasa smart plugs and 6 Echo devices never seem to have any problems connecting to my mesh of 3 AP960c's and stand alone (for now??) 2927ax...

Just sayin'

(not that I haven't had problems in the past, particularly with 5GHz on AP903's and Apple devices. Fortunately those issues appear to have been resolved)