Get ASUS at your peril. I'm all for giving companies second (or even third chances) but ASUS constantly failing and October to November 2024 when they had critical vulnerabilities installing malware on the router was hilarious.
You may be asking "BUT BUT MERLIN!!!" yeah I also think it's a bad idea to leave your whole networking security to one person on the internet. Imagine RMerlin dies tomorrow that leaves your custom firmware router basically useless until the next ASUS vulnerability is found (give it a couple months lol).
It doesn't have to be the Flint 2 but just get something capable of running OpenWRT. Right now a good shout is the Linksys SPNMX56 which can be had for around £40. It's Dual band Wifi 6 that got OpenWRT support this week. Yeah it's a tad of a learning curve but atleast you then know security is a non issue for as long as that device can still power on.
OpenWRT ofcourse has Option 61 support. More info here:
https://www.reddit.com/r/openwrt/comments/134mb08/sky_fttp_with_openwrt_router/
Also if you do go GL Inet instructions in post 23 here:
https://forum.gl-inet.com/t/sky-dsl-dhcp-61/46043