Where circumstances permit, I much prefer installing a transfer switch/subpanel combo, and I avoid pre-wired "solutions". To my way of thinking, they either constrain your options or you end up rewiring and reworking them and get little or no benefit from the prewiring.

Can you re-run those circuits to a subpanel, or is that not feasible given your placements? Reliance has another series based on Siemens subpanels that is a lot cheaper than the model you are looking at, even after you buy breakers.

Also, if decide to discard included wiring and replace it with PVC conduit and new conductors, don't assume that you can match the sizing of the conduit and wires. Those transfer switches that put a whole bunch of small circuit conductors in a small-diameter flexible conduit get past UL certification because they qualify as a short (If you try to do the same thing for a longer distance, it'll kick in ordinary fill limits (40%) and, with ten or more conductors in a single raceway, you'll have to derate the wire sizes too.

Consider adding an inlet, breaker and interlock kit to your existing panel. I don't like interlocks much, but it seems as safe an alternative as either of the (so called) transfer switches you mention.

I believe that a manual transfer switch should go CLUNK when you pull the handle and have blades that provide a visible open. I don't trust interlocked breakers as typically used in residential and light commercial appplications.

Here is a link that might be useful: here are some interlock kits