what wattage should i use for my iPhone 8 ?

iPhone 8 recommendations are for 5 watts, or for 18 to 96 watts for rapid charging.

Older 10 watt and 12 watt iPad chargers also work.

About Apple USB power adapters - Apple Support

Fast charge your iPhone and iPad - Apple Support

I tend to use the 18 watt and 30 watt chargers, as they’re both small and charge fairly quickly, though that’s with other recent iPhone and iPad models.

I would absolutely stay with Apple chargers for USB-C connections, as there are some trash-grade USB-C chargers available.

Yes, you can use the 12 watt iPad charge cube.

Some of the available info on batteries is certainly good and valid. Prodigious quantities of drivel have also been published around batteries and battery tech. Lots of scams, “sponsored content”, and such around, too. Some of the published tech content is simply stale and seemingly hasn’t been updated since the era of NiCd batteries, just as storage suggestions that arose from the earlier parts of the hard disk and floppies era are less than entirely applicable to SSDs... And the increased user engagement from providing bad info is the fundamental foundation of various social networks. Trolling and conspiracies and Internet Experts are free (even around here, and quite possibly including me), while reality is increasingly paywalled or pigeonholed or search-buried. Welcome to 2020. But I digress.

Check with the manufacturers for what information is available.

Some battery-related reading directly from Apple: Batteries - Apple