I think the DIY is "it's a messy job".
I use a Tyvek suit, gloves and goggles the few times I've done undercarriage cleaning. I used degreasers (simple green, Meguiar's red super degreaser, and also a petroleum based degreaser for baked on stuff), brushes, rags, and hose pressure hot water. On my E39 M5, although it had essentially never been in rain and of course never a winter road, it did suffer from the infamous power steering hose leaking plus general road dirt over the 15 years I owned it (sold on BaT in 2018). I spent maybe 2 hours with it on a lift cleaning it up, but it wasn't too nasty to begin with fortunately given its 66k miles.
The best DIY is start from brand new and never let it get too dirty!