Please note - I am not a mechanic or expert in anything.
The long clear motorway runs will be good (depending on the overall gearing of the car) but your regular run when clear could also be used to clear the car and have a bit of fun (not stupid driving of course) it's also about the revs and acceleration, have a good volume of air being sucked in and more fuel being pushed through, that can be done at lower speeds than motorway boring cruising. I think cars that with a steering wheel, manual gearbox, brakes, suspension and good tyres should sometimes be driven on B-roads in a way that will exercise them and a small car (and it doesn't need to be a GTi type) can be more fun on these roads than a larger wide-bottomed quicker/faster car.
It does wear the engine and car more at those times but it's good to stretch (and test check) the car (when/where safe and OK to do so). The engine's just a big air pump anyway, which also brings me to my other saying, GIGO for computers but its SISO for cars, especially diesel but now also perhaps also more modern petrols. This means having the air filter and induction clean, regular, timely, thorough (possibly only possible if DIY) engine oil and filter changes. I also favor keeping on top of the plugs and cleaning MAF and throttle body when required.
Doesn't the Owner's Manual have anything about warming the car up then driving it to clear it out, I thought it had info about short journeys effects (and on the battery), IIRC the Mk1(?) and 2(?) Driver's Handbooks had loads about economical driving, even down to electric use effect on alternator and mpg.
As I put I think it takes a couple of tankfulls, not top-ups of the fuels with higher cleaning additives (and octane) to perhaps see (feel) any benefits. For the diesel lot, and perhaps your car but I know even less about GPF, I think a couple of tankfulls before/during MoT and before/after a service might (no printed evidence to give) be helpful.
My wife used to say her car felt better after I had used it and we both have felt that the Fabia goes better after it has been serviced.
Be interesting to see how you get on, good luck.