The a6600 and the a6400 pretty much share Sony's current state-of-the-art APS-C auto focus and both are weather sealed, but the a6400 lacks IBIS. The a6500 has IBIS but the previous generation AF. Personally I would pay more attention to those factors over the battery.

BTW if you do decide on full frame, MBP has a good selection of "like new" A7c models for $1469, which was the best value I found when I was looking for myself.

https://www.mpb.com/en-us/product/sony-alpha-a7c

You are most welcome, happy to help.

Sony fullframe models also have the option to shoot in APS-C mode, giving you that extra reach, and Sony also has Clear Image Zoom (in jpeg only) which will give you an additional 2X magnification without image quality loss . So with fullframe you have the best of both APS-C for extra reach and the best of fullframe for better image quality of the larger sensor.

As much as I loved my A6500 for its compactness, my fullframe images are better image quality, with shallower depth of field when I want it, and better crop ability in post without losing image quality. As a casual hobbyist it may not matter that much to you.

The compact fullframe Sony A7C is not much larger or heavier than some APS-C Sony models. Comparing the APS-C A6600 to the fullframe A7C, the A7C is slightly larger and taller but the weight is basically the same: A7C: 124 x 71.1 x 59.7mm, 509g. a6600: 120 x 66.9 x 69.3mm; 503g. Both do have EVFs.

Cheers and best to you.

IMHO in your situation I would trade-in/and or sell all your APS-C gear. And then get a good used Fullframe Sony A7III or A7C, both share the same 24mp Fullframe sensor and have excellent still and video performance, and then shop for good used third-party native E-mount lenses that will give you great image quality, but wont break the bank.

With your trade-ins/or sales of your APS-C gear, and good shopping ,you could stay within your budget.

https://www.sony.com/gh/electronics/interchangeable-lens-cameras/ilce-7m3-body-kit

https://www.sony.com/gh/electronics/interchangeable-lens-cameras/ilce-7c

There are over 200 native E-mount lenses from Sony, Sigma, Tamron, Samyang/Rokinon, Zeiss, Tokina, Viltrox, Meike and others, in all budget ranges. And you can find them new or used. Try shopping reputable used gear dealers like B&H, Adorama, KEH, MPB, Robert's, Allen's Camera or even Ebay. Stay away from Abe's of Maine, a dishonest grey-market dealer ,known for bait-and switch, hidden fees, and non-compliance with returns and refund policies.

https://en.wikipedia.org/wiki/List_of_Sony_E-mount_lenses

https://en.wikipedia.org/wiki/List_of_third-party_Sony_E-mount_lenses

FYI, after moving over from pro Nikon and Canon (40 years both brands) to Sony, I have owned Sony APS-C A6500; and Fullframe A7RII, A7RIII, A7III, A9, and currently own A7RIV, A1, A7SIII. I currently own 13 fullframe native E-mount lenses covering 10mm to 600mm from Sony, Sigma, Tamron.

Cheers and best to you.

Before you succumb to a software GAS attack, assure you're being effective in actively managing and overriding the default settings that LR gives for both Noise Processing and Sharpening. Ideas and examples are presented here:

Basics of noise processing

Basics of Lightroom Sharpening

I have an older Topaz DeNoise as a 2nd step to LR's NR processing, for higher ISO images. I sharpen both before and after, only in LR, for images sent to DeNoise, using the techniques discussed in the link above.