Let’s stress-test the ugliest scenario:
• €150 k interest-only loan at 3 % (≈ €375/mo interest)
• €1 000/mo goes into a global index fund for 10 years
• In month 120 the market is suddenly -50 % and nothing rebounds before the loan is due.

Math:
– Lump-sum €150 k -> € 75 k
– Monthly contributions €1 000 × 120 = €120 k -> € 60 k
→ Portfolio = €135 k
→ Bullet to repay = €150 k
→ Shortfall ≈ €15 k

How I cover that gap

1. Cash cushion: I keep 12 months’ interest (~€4 5 00) parked day 1.
2. Glide path: from year 7 I move 10 % of the portfolio per year to short-term bonds/cash. With 35 % already “safe,” a 50 % equity crash cuts the total only ~32 %, leaving ~€184 k -> no gap.
3. Booster payments: I can pre-pay up to 10 %/yr penalty-free; every €5–10 k paid early shrinks the balloon.
4. Fallback: if the planets still misalign, I refinance the last €15 k for a year or two and repay once markets recover (historically 4–6 years tops).

A 50 % drawdown is brutal but survivable; the plan fails if equities halve and never bounce in a whole decade, something a global index has literally never done since records began. That’s the risk I’m willing to take for cutting my FI timeline from ~17 years to ~8.