my RB5009 never over 512MB of use even AdGuard Container is running.

if you have a lot of connection, yes, RAM is important for keeping connection information (Dest IP, Src Ip, Dest Port, Src Port, Protocol, etc...)

during my test, one IDM download from Google Drive (8 parallel download) do increase my RB5009 RAM usage, this happen connection is IPv6

Yes it can😁

Great! RB5009 can handle IPv6 very well even Mikrotik IPv6 don't have FastPath, CPU usage around 60% when test, that includes AdGuard container running.

Make sure TM did change 2.5G ONR/ONU for you.
And if later u are not satisficed with the ONR/ONU given then u can buy the custom ONU from Anime4000.

I just get my 1G FSU on early Feb,after report and technician visit,now so far so good. Although i not sure why i cant hit full upload to Google Drive. (I can hit full speed with multiple upload tasks)
According to Anime4000 said Dlink 2.5G black combo box and Skyworth 2.5G black combo box is also not fully stable. (And somehow buggy)

Need to remember, ISP equipment never been safe from hacker and lacks of general update, often use very old stuff, these device so cheap and security maintenance never a thing

thing is, well secured equipment never need to reboot, but device feel sluggish can determine has been infected, a hacker use some it's resources for their bidding.

hell, even you disable ISP Management, ISP still have full control which is very bad