Well for a start it's good that you are thinking about this stuff now. I wish I had been!
The US looks well placed for the next few years aside from the huge debt pile and overpowering dollar strength. You likely won't go wrong in switching to the S&P over the time period. If you had any spare cash to save I'd think about opening a Stocks and Shares ISA because then you can build a base of investments that are tax free and it will allow more diversification into other asset classes. I suspect digital currencies and blockchains will be playing a far more important role in the next 20-30 years so I'd want scope to be able to gain exposure to that. I suspect company administered schemes will be very slow to move in that direction.
S&S ISA next thing to sort out. I jumped on Trading 212 this year and put money in their Cash ISA which looking to drip feed into a S&S ISA with them. For that I was looking at S&P 500 VUAG as my only holding and perhaps play it safe with my pension and keep it just an index fund. So many options but then the ISA would fund family milestones in the future so might dip into it in 10/15 years time.