Jesus man, I've never seen such dithering... Near 18 months and you've still not done anything.
As
@Avalon said, swerve anything Asus unless you're buying cheap/second hand and know for a fact good open source firmware is available to slot straight onto it. Is there any reason you've not just put the Virgin Super Hub 3 into router mode and tried with that? With such basic requirements (again, 25 clients is nothing lol) it'll likely do you fine, and the wireless is a damn site better than on your old Asus for sure. If you start bumping into limitations with that, *then* I'd suggest looking to spend money.
Regarding content blocking and timed access restrictions; it's always tricky to solve a parenting problem with a technological solution. As it happens, the SH3 has time restrictions available on a per MAC address basis, so you're covered there. Go to Advanced Settings > Security > MAC Filtering after signing in at 192.168.100.1.
Look at the kids' devices too. Windows allows you to set up a 'Family' and govern content and time restrictions for children's accounts, iOS and macOS offer this with Screen Time, and I'm sure Android will have an equivalent. The SH3 will cover the Internet access aspect at least.
As for actual content blocking, like stopping porn/drugs/gambling/violence or whatever, you could run AdGuard Home and tell it to restrict your kids' devices. You can even use it to disable access to websites, using a block list and/or pre-provided services like TikTok, Steam, Discord, Cloudflare, Amazon, Netflix, Skype, eBay, Facebook/Snapchat/Instagram etc etc. Very handy, and free. You just need to a device to run it on - so if you really want to spend cash get a Raspberry Pi for about £50 and install it on there. Learning Linux and such will keep you busy while we're in lockdown.