My questions:
i) I’m afraid of having a diversification problem and over-indexing in tech stocks if I buy a standard all world ETF (VT/VTI) - is this a legitimate thought? Which alternative ETFs would you recommend that exclude tech?
I wouldn't worry about it. VT is "only"
15% tech, which really isn't huge. But:
you should sell your employer stock and invest the proceeds into the diversified fund. (I'm in a similar position, I sell my stock as soon as I get it. My much more experienced manager has given me some great stories about former coworkers who kept all their tech stock, and then suffered when it inevitably dropped for a few years.)
ii) I opened an account with DEGIRO to find out that several of the US ETFs are not available anymore, including VT. Given the current selection, which ETF would you recommend?
EU regulations prevent EU residents from buying US passive products (funds/ETFs), because they require specific documentation that US funds/ETFs aren't providing (the so called PRIIPS regulation). You'll have the same problem at most other brokers, so long as they have an EU domicile (which includes IB). I don't know of any good brokers who can get around that...
VWRL (which has a few variants denominated in various currencies, but they're all the same underlying fund) is the closest equivalent to VT and is IE-domiciled, the key difference is that VWRL only has large and mid-cap, and doesn't have small-cap (perhaps you can find a second fund to add small-cap exposure).
(I'm slightly contrarian and underweight the US, so I actually buy small amounts of Europe/Pacific/Emerging market funds on top of the total-world fund - but that's only worth it if you like juggling a complicated spreadsheet every time you invest.)
iii) Alternatively, would it be worth it to open an account with another broker (thinking of IB) given my current volume? If so, which ETFs would you recommend?
You should compare fees, and make the decision based on that. You should have access to the same funds/ETFs at IB (e.g VWRL) - as mentioned above, you won't have access to VT at IB. IB have a slightly horrible user interface (not that hard to learn though), but they tend to be cheap and offer cheap currency conversion, which can be useful if you ever need to buy a non-EUR denominated fund, or if you move, etc.
iv) Do I need to include bonds given that I already invest 7% of my salary in a non-volatile pension fund?
This is something you have to decide for yourself, but personally I wouldn't at your age, especially if the pension is non-volatile. As you get older/closer to retirement you probably would want to boost the bond percentage. However: this only works if you're sufficiently unfazed by large market drops, the typical scenario is that people overestimate their ability to survive drops, take on too much risk, and then sell when scared after a large drop. If you can avoid that, 100% stocks is fine. If not, some bonds won't harm you much.
v) Is there any other advice that you would like to give to your 25 year old self?
I'm not much older so don't have much advice...
But if you happen to be in Dublin, I hear 3fe have good coffee... But that might be bad advice, you should spend your money on investments not coffee.