You surprise me, I have all of the Trend micro features on, and it does not slow my network down at all and if it does it is so minimal I cannot detect it.
I also use flex QOS (Merlin plugin) and Merlins latest firmware.
You do not state your isp speed however.
Over 500 probably you don't need any QOS, 250-500 I would use Flex, Under 250 I would use Cake.
Mine is supposed to be 500/30 but on speed test I get 550/35 with Trend fully on and skynet on (Merlin plugin). WIth Flex on we get literally zero rubber banding or spikes in online games, and there are 5 of us online all day long.
I would 100% not disable UPnP, this will cause you more problems than it solves, the security risk of having it on is based on out of date, misinformation, and you will have trouble when you play games or use other services. I believe ASUS products are unaffected by 10 year old misinformation on uPnP risk. Ask yourself if you really want to manually open ports everytime you need to do something.
For now I would leave smart connect as is, with one SSID, however you can try separate ID's for 2.4 and 5ghz in the future if you feel the need, some do , some don't. I personally use Smart Connect with no issues and one SSID. I force my own pc onto 5ghz by blocking it on 2.ghz with mac filter.
The other devices choose where they go themselves based on which room they are being used in. I do block my google speakers off 5ghz though as they are bit sketchy on 5ghz, same way with mac filter.
2.4ghz I would set to 20mhz channel 11 or 6 depending on your neighbours; why 20 and not 40? Stability for me - 20 is slower but performs better in crowded areas, 40 is faster but less stable .
5ghz I would set to either 80mhz or 160 (but only if you devices that support 160 which are very few and far between) I use channel 64 with great success as no one else in my street can get this channel.
I would agree that DNS servers 9.9.9.9 and Cloudflare 1.1.1.1 are good choices. (or 1.1.1.2 (No Malware)) or 1.1.1.3 (No Malware or Adult)
I don't think there is any privacy issue with Trend; I would rather all the bad websites and ips attacks are blocked personally. Trend are a trusted company of some 25 years and are not interested in what you look at online except for security purposes. Please don't think you ever have privacy online anyway. The likes of google , microsoft , facebook and many others track you even if you do not use their services. What makes you think you are so interesting in their 250+ million user base. Although when I first got my router I did have the same worries, but the users here have reassured me Trend is safe.
Congratulations on buying the best router on the market however, I would strongly recommend Merlin over stock firmware and reading
@L&LD s guide on how to set up this router, but each to their own. Enjoy your router.