Hi, I’ve been in a similar situation to you, having bought what I thought was a ‘cheap’ 996 and then got stung for a similar amount of repair money not long after.
The way I think of it is this-
Yes in hindsight we should have been more careful pre-purchase. A Porsche specialist inspection could have spotted things like the corroded rads, air con pipes, clutch life etc and paid for itself many times over. Or, buy from a dealer with a warranty next time, just don’t expect it to be so cheap.
Things like brake wear is just common wear on any car, it’s part of the game. And once you purchase a used car with worn brakes once you’ll be much more observant on your own inspection time!
No I don’t expect a 996 to cost £4k each year to run. It might be wise to budget for £1-2k. You’d be unlucky to have another £4k bill next year.
No your car won’t be worth £8k more now but it will help if/when you decide to sell having had a number of the ‘big’ items done. A C4S with ims/rms/clutch/rads/brakes done and 80k ish miles is a desirable car.
The way I’d look at it is this. Car ownership is not ‘free’. Usually depreciation is a factor no one thinks about. You could have bought a VW Golf, not had to spend a penny on maintaince other than minor services and still taken a £3k hit per year in depreciation. However a 996 hardly depreciates at all, in fact might appreciate over the next few years. I seem to remember reading that Porsche 911s are the most depreciation-proof cars ever. So you have that in your favour in terms of total ownership cost.
Lastly, personal opinion but I think once interest rates come down in the next couple of years, classic cars (including modern classics like 996s) will appreciate again.
Hang in there, put some miles on the car, book a trip in it. Good memories are the perfect antidote to bad bills. On your death bed you’ll think about the memories not the bills.