<=1.5kg solution for long-zoom action photography (esp. BIF)?

I'm researching solutions for long-zoom action photography, especially BIF (birds in flight). In particular, the solution should be able to capture a kingfisher flying and catching a fish.

My general requirements are:

• The total weight of the camera and the lens should be <= 1.5kg.
• The max focal length (35mm equiv.) should be at least 600mm, and a greater amount is much appreciated.

Currently I know of three categories of possible solutions:

1. Sony RX10M4 (1"-sensor, 24-600mm equiv.)

The RX10M4 weighs only 1.1kg and I know of a 95-year-old man who used to use all kinds of full-frame long-zoom lenses but ended up using the RX10M4 alone for daily BIF at local parks because he is no longer able to carry those full-frame lenses. Although I'm not 95 years old yet, I'm a lazy Garfield and appreciate convenience so I limit the solution I'm seeking to <=1.5kg.

I also actually see people using the RX10M4 to photograph kingfishers flying and catching fish - it works. One of them complained that the camera has only 20MP, compared to some modern high-end DSLRs which have at least 45MP.

The RX10M4 costs 10400 Chinese yuan (US$1640) on Tmall.com (China's de facto Amazon).

2. An APS-C body + the Tamron 18-400mm APS-C lens (27-600mm equiv.)

The Tamron 18-400mm F3.5-6.3 Di II VC HLD is a lens with an EF-S or Nikon F (DX) mount that weighs 710g and costs 4100 Chinese yuan (US$647).

Currently I know of two possible APS-C camera bodies that may work with this lens:

• Nikon D3500
  The D3500 has 24MP, a little greater than the 20MP of the RX10M4, and a max focal length of 600mm when working with the Tamron lens. If a 600mm D3500 photo is cropped to 20MP (to match that of the RX10M4), the photo actually has a focal length of 657mm now. The camera weighs 365g (or 1.075kg with the Tamron lens, about the same total weight as the RX10M4). The camera costs 3400 Chinese yuan (US$536), or 7500 Chinese yuan (US$1183) with the Tamron lens.
• Canon 90D
  The 90D has 33MP, much greater than the 20MP of the RX10M4, and a max focal length of 640mm when working with the Tamron lens. If a 640mm 90D photo is cropped to 20MP (to match that of the RX10M4), the photo actually has a focal length of 816mm now. The camera weighs 701g (or 1.411kg with the Tamron lens). The camera costs 7900 Chinese yuan (US$1246), or 12000 Chinese yuan (US$1892) with the Tamron lens.

3. A Micro Four Thirds body + the Panasonic 100-400 Micro Four Thirds lens (200-800mm equiv.)

The Panasonic Leica DG Vario-Elmar 100-400mm F4.0-6.3 ASPH Power OIS is a lens with a Micro Four Thirds mount that weighs 985g and costs 7550 Chinese yuan (US$1191).

The Panasonic Lumix DC-G100 seems to be a Micro Four Thirds camera body that weighs no greater than 1.5kg when combined with the said Panasonic lens (352g camera + 985g lens = 1.337kg). It has 20MP and costs 4100 Chinese yuan (US$647) or 11650 Chinese yuan (US$1837) with the Panasonic lens.

Summary

Currently I have 4 options in mind:

• Sony RX10M4: 1" sensor, 20MP, 24-600mm, 1.1kg, can do kingfisher BIF, 10400 yuan ($1640).
• Nikon D3500 + Tamron 18-400mm: APS-C sensor, 24MP, 27-600mm (or 657mm if cropped to 20MP), 1.075kg, 7500 yuan ($1183).
• Canon 90D + Tamron 18-400mm: APS-C sensor, 33MP, 29-640mm (or 816mm if cropped to 20MP), 1.411kg, 12000 yuan (US$1892).
• Panasonic G100 + Panasonic 100-400mm: Micro Four Thirds sensor, 20MP, 200-800mm, 1.337kg, 11650 yuan ($1837).

Which of these options do you think is best for BIF and why? And is there any other option I missed?