Just to give you an update: For the first step, I opted to order that device: . From what one could read, this will do the trick, it's ready to use and affordable. The hints KE7ER gave are indeed very helpful, I might try that at a later stage - but as you stated, running coax to that circuit board may not be the best solution and even if everything goes well there will be unwanted signals in the waterfall. The advantage I hoped for is not to worsen the receiving signal - so every solution has its disadvantages. Using that kind of switch seems to be the less-risky option to get a nice panadapter along QMX+. And, maybe one day I/Q mode will be usable in parallel to normal operation rendering that approaches superfluous.
