Interesting DM, that the comments about the video do call into question some of the ideas proposed.
The moral of the story is trust the folks here that have much practical experience with many different setups.
Trusting a single Youtube video as the defacto source of true information is a fools game.
In addition the best ways to get accurate and efficient help is
a. describe your requirements without referring to solutions or configurations but in use cases.
b. /export hide-sensitive file=anynameyouwish
c. draw a diagram of your network
This works when you specify routing table. But it doesn’t when you don’t. It’s known problem and it doesn’t depend on RP filter. If you check routing diagram, you’ll see that packets from local process (K) first go to “routing decision” and it works with main routing table. You can use mangle rules to set different routing mark for packets (and have them use different routing table), but it won’t help with router’s own output, because that’s in “output” box, which is after already failed routing decision, so it won’t get there. You won’t have this problem with forwarded packets, because they start at (I), then there’s prerouting with mangle rules and routing decision comes after, so packets already have correct routing mark.