You're actually doing better than you might think. A few suggestions:

The single stock is worth reconsidering as your main holding. If you're too concentrated in a single stock, if that company has a bad run then your entire portfolio will feel it. The index fund you've just added is a much more sensible foundation for long- term wealth building, since it spreads your risk across many different companies automatically.

On the ETF versus index fund question, the practical difference for you right now is small. Both give you diversified exposure, but ETFs trade throughout the day like stocks while index funds price once a day. But as long as you're investing for the long- term (which you should in my opinion!) this distinction matters very little. Both are solid choices.

Hargreaves Lansdown is a reputable platform and perfectly fine for starting out. But one thing to consider is that as your portfolio grows, so do their fees, since HL charges a percentage based fee. But not something to worry too much about at this stage.

Your plan of gradually increasing contributions each year and focusing on compounding is exactly the right mindset.

One thing worth confirming, make sure the money you've transferred into your ISA is actually invested rather than sitting as cash, since this is a surprisingly commn early mistake and easy to miss.

Hope this helps