A brewery nearby (buxton brewery) does several beers (some as colabs with omnipollo) as kettle sours with lactose. They do a lemon meringue pie as one example which I had recently. I know this isn't a berliner, but it is pretty much the kind of thing you are aiming for. Commend you for tackling it.
The process has already been described. Mash. Heat to pasteurise (or don't). Kettle sour. Boil (or heat to pasteurise again). Ferment with sacchoromyces. Do your thing. I'd definitely kettle sour so you can add the lactose at the end of whatever boil method you choose to use.
I've had good success with controlled souring (pitching a large healthy proven culture) and I've gotten away with just fly sparging hot (80C at least) so far as a poor man pasteurisation prior to this. Benefit of the proven culture is predictable outcome, clean souring with a fast turn around.
I've found sharp fruits help add more complexity to a kettle sour which can often be too simple/clean (though this is definitely desirable sometimes).
As to the beers I've had (sour, lactose, fruit) I find them pretty unpleasant. I like sour beer, but I'm starting to prefer them light, crisp, clean and refreshing. When they start to get too complex there is often too much fighting amongst the flavours. While I can appreciate the intent and the execution they are often a curiosity for me. I've got approx 50 bottles of a bretted cherry kettle sour which while a brilliant colour, is just a bit too beery (hopped too much) and with too much spice, clove floral antiseptic brett notes for me. I've also got approx 50 bottles of a lemon lime hibiscus gose which is much more drinkable, but I wish I'd left plain.
