As you say, an alarming experience.
Difficult though to find an answer. As your experience demonstrates, the receipt of a RIS does not eliminate the risk, and I'd suggest that radar is not a viable methodology. After over 100 ears of development a system has evolved in which the major means of separation is procedural, using controlled airspace, with aircraft flying along assigned tracks, separated by height and timing, rather than by a process of radar control.
A look at the process for vectoring aircraft into Heathrow and Gatwick demonstrates the system that has evolved. It's not a reactive system, it's a procedural one. Aircraft are placed in stacks, separated by height, and are then pulled from them in sequence onto pre-determined tracks. They form a very distinctive pattern on the screen, making it easier to spot an intruder.
If the use of reactive systems like RIS or RAS were a viable alternative to procedural systems the airlines would be pumping money into their development, It isn't and they're not.