I appreciate I'm coming across as a bit potato obsessed
... I just like to make sure my family are happy with home cooking!
I used to know an East German who'd moved to the UK. She thought she was a potato expert because growing up their main food was potato soup. It completely blew her mind to see how seriously the British took potatoes, with supermarkets having half-a-dozen different varieties for mashing, roasting, salad etc. [at one stage there were specific laws requiring potatoes and melons to have the variety declared on the packaging, but not other produce]
Start in cold water to ensure the potatoes cook through without the outside being watery.
Nah - that's amateur talk. Mashing potatoes is like brewing, all about manipulating starch. Gluiness is caused by starch granules bursting, and you prevent that by gelatinising it and then recrystallising it, which needs two stages of cooking separated by cooling.
Cut potatoes into inch-thick slices as consistently as you can - variation in thickness means variation in ideal cooking time, and you want all your spud volume to be just on point. You can cut thinner but need to reduce cooking time. Rinse them in cold water to get rid of any loose starch and pectic enzymes.
Start with unsalted water that's at 80°C - mixing boiling water and tap water about 4:1 should get you there. Adding your spuds should take it down to about 74°C, and keep it there for 30 minutes (for 1" slices or 20 minutes if your spud slices are 1cm) - this is below simmering temperature. In this phase the potatoes absorb water and the starch swells and gelatinises.
Drain the spuds and run them under the cold tap until they're cold. This causes rapid recrystallization of the starch in a process known as retrogradation and leaves it largely insoluble, so it can no longer form glue. You can leave them in the fridge at this point until needed.
Then simmer for 5-10 minutes in salty - like seawater-salty - water until they are soft to a knife. Simmering at this point breaks up the cell walls so the cells separate without bursting. Don't overcook them as otherwise the mash just ends up watery. Run them under the cold tap until cold again, then put them in the empty pan over heat just to dry them a bit whilst you assemble the bits for the mashing phase.
Retrograding the starch is the crucial thing, it vastly improves the quality of your mash and once you've done it a couple of times you no longer need your Thermapen and can do it "by eye".
Our family tradition was always to add nutmeg to regular mash, but I have been converted to adding Dijon mustard when having it with sausages.