Only save in one ETF. I would go for your physical replication AND choose a much cheaper ETF (max. 0.2% TER). Amundi already offers the MSCI (without emerging markets) for 0.05% TER.
That is 9x cheaper than your chosen ETF (0.45% to 0.05%)

ACWI = with emerging markets
ACWI IMI = with emerging markets and small caps

Keep it simple. You're already putting far too much effort into this than necessary.

Grandma can either make a standing order to the clearing account, give you the money in cash and you invest it, or or.

Sector ETF for long-term investment ... Better not. How do you know that the sector ETF will still be running in 20 years' time?
@MoneyISnotREAL Which one do you mean exactly? I can't find one with 0.05%. Maybe you can link it here.
@Aah91 $PRAW $PR1W

As I said, the world ETFs usually have a TER of around 0.12 - 0.2%.
I myself have the one from HSBC $HMWO with a TER of 0.15%.

Amunid also has an All-World-Country (is like ACWI) $WEBG with 0.07% TER.