I have got both the Legacy game and Ultimate Collection, and I have gone back to UC only because it is more stable and like you I had started getting pink soup in Legacy.
As far as I am aware the GRM is not yet designed for the Legacy game - but if it could be used the path would be Programme Files/EA Games/The Sims 2 Legacy/EP9. Mainly, GRM edits the Graphics Rules and Video Card SGR files that are found in the game's programme files. The actual location of these SGR files are in Programme Files/EA Games/The Sims 2 Legacy/EP9/TSData/Res/Config. Before GRM came along we used to have to edit these SGR files manually to make the game work on more modern machines (as well as other tricks) and I imagine a lot of people still do.
As far as I can make out, it isn't supposed to be necessary to edit the Legacy game's Graphics Rules or the Video Card SGR files, and personally I wouldn't touch the Graphic Rules SGR file just in case I screwed it up. However, one think you might try is to add your Video Card to the Video Card SGR file. If you look in Documents/EA Games/The Sims 2 Legacy/Logs and open the config text document in there you will see amongst other things the details of your video card and underneath whether it is included in the database. All that means is it listed in the Video Card SGR file in the programme files. It will probably say "not found in database" next to it. Theoretically that shouldn't matter, but you could try adding it to the Video Card SGR file (edit it in notepad) - but make save a copy of the original just in case. I think that one of the problems with UC is that if your card isn't in the database the game assumes your card isn't good enough and the pink soup is one outcome of that - maybe adding it to the Legacy database will help.
Another funny trick I found is that if I got pink soup in my usual save, but then loaded up a lot in a neighborhood I don't play (e.g the Goths in Pleasantview) and that looked ok, if I went back to the pink soupy save, sometimes it righted itself.
Also another trick, is to save regularly, shut down the game regularly, reboot your computer, delete the game caches (group, accessory) and that can help for a bit.
And finally take a look at Lazy Duchess's Legacy game fixes - they do some great work, and their TS2 extender might help the pink soup issue.
Sorry if you know all this already - but thought I would say it, just in case any/some of it is of help. I would not be surprised if people a lot cleverer than me who helped UC playable all these years will be adding more Legacy fixes/solutions to what EA can't yet manage themselves.
