Using power adapters to charge Fenix 6

If that's a 61W PD (Power Delivery) adapter, which I assume it is, rather than a regular USB power supply then I would not risk it.

A phone charger would be a safer bet, especially if it has USB-A ports rather than USB-C. I'm not sure about converting USB-C to USB-A for the watch. It might be fine, but I honestly don't know. The issue is not with the connection type as such, but the voltages available and the negotiation of power demands between the watch and the supply.

I wasn't sure if the watch could detect and throttle the power input like iPhones do? Hopefully someone can confirm.

It does, but we don't know if a super-high output charger might overload the "throttling" circuitry or not. Probably not, but I wouldn't risk it myself. Garmin's official recommendation is to charge using a computer USB port, but it's been demonstrated that it's also perfectly fine to use a standard phone adapter. I've plugged my watch into 1amp, 2.1amp, and 2.4 amp phone chargers, and it always takes exactly the same amount of time to charge the watch regardless.

My math might be wrong (a little hung over today ) but I think 61W at 5V means it's an 8amp charger, which would be 3-4x more power than a normal phone charger. That is, unless 61W is the rating spread across multiple usb ports on the charger, where in fact each port individually only has a rating of, say, 15.25 watts for each of 4 ports (3 amps per port), which is still a little high, but probably safe.

A battery bank is also fine, but sometimes the watch doesn't draw enough current so the battery thinks nothing is plugged in and turns itself off after a minute; some do, some don't. My power banks have 2 or more USB ports, so I'll usually plug my phone (or another device) into one of the other ports to keep the battery from turning off while it's charging my watch.

I use this all the time for charging various devices, but the 60W PD circuit is reserved for running my XPS 13.

Tablet, phone, power banks and watches if need be are only juiced up from the standard USB-A ports. Usually, though, my watches (four of them) are charged from my laptop as it's convenient to check for updates while charging.

In THEORY this should be safe. In practice the problem is the adapter to get from USB-C to USB-A/Female, which is what the watch's cord uses.

I wouldn't try it because it is that adapter that has to be wired correctly, and if it puts much more than +5V on the watch's power pins (which PD can do) it is nearly-certain to release all the magic smoke.

I've never seen anything like this. Could you tell me the manufacturer? 

Lots of companies make those. Aukey or Anker, being two respected companies each have their own versions.