OP here — naming it 'the setup tax' has genuinely helped me stop panicking about the numbers. Ours went mostly on the rental deposit + agency fee combo, then furnishing an empty flat. I'd tell anyone to budget a whole extra month's costs on top of what they expect.
Deposits, agency fees, furniture, appliances, duplicate subscriptions while I closed old ones, a hundred small 'new here' purchases — my first three months cost nearly double what I'd budgeted for 'normal' life. I wish someone had named this 'setup tax' for me. How big was yours, and where did the money actually go?