I'd say the approach is potentially vulnerable, but the tech isn't quite there. The modern approach to password cracking is to take a huge dictionary, and run permutations on it, like change a's to @'s, capitalizing first letters or adding numbers in the end. Any cracker worth their salt will have something like "add _netflix" as a permutation, too. I don't think that anyone would have "NutFlex" in there, yet, but it's possible if one of them stumbles on your leaked password from somewhere else.

As for "basic text", do you mean like .txt's? And do you store the entire password there? We do have viruses that scan for crypto wallets and it's seed phrases already. It's not too far fetched to imagine one that would cross-match any txt's contents in the system with browser's saved logins.

The most glaring issue I see is that the bastardization is effectively part of your password. With 1000+ passwords it's going to be easy to forget (was it nutflix, sneedtflex, nyetflex or something?) and it's going to be hard to find it if you don't manage the codes properly. I recently had to scan over every single of my password manager entries (forgot a 100% random login, password and domain), and let me tell ya, It wasn't fun.

You could possibly switch to a "client-side salting" approach, having a strong consistent password in you head, and storing a short but truly random suffixes for each service. e.g. text file named "Netflix" containing something like "T3M#f" and the final password would be something like "hunter2T3M#f". At least that's what responsible sites do to protect people who have simple/matching passwords. You could even store those suffixes somewhere semi-openly, like in a messenger as messages to yourself. But at that point, it's probably easier to go with a password manager. Though that's an option if you don't trust those.