I also shoot a lot of wildlife and BIF at the San Diego National Wildlife Refuge (both locations), Ramona Grasslands, the beaches, San Diego Zoo, Safari Park, SeaWorld, Discovery Center, and the East San Diego County boondocks. I rented the Tamron, the Sigma Contemporary, and the Sigma Sport for one week each. Each was tested by wandering around carrying the camera and lens in hand, ready to shoot at a moment's notice. I don't use a tripod except for time lapse photography over several hours, so how I felt after 8 hours wandering around carrying these beasts was critical.
Here's my complete review:
https://www.uglyhedgehog.com/t-458752-1.html

You might consider the new lens out by Nikon. The AF-S NIKKOR 500mm f/5.6E PF ED VR, a Fixed Focal Length Super-Telephoto Lens as it is very small for a 500mm lens. I have read good things about it. Remember your lenses are your long term investment. Quality glass you will keep and the camera body will be what you will update from time to time. Another option, although bigger and heavier but cheaper would be the Nikon AF-S NIKKOR 200-500mm f/5.6E ED VR Lens. It is similar in price to the Tamron and is a very sharp lens.

A few months ago I upgraded my Nikon 3300 to the D7500 which I'm very happy with. I've also rented the Tamron 150-600 G2 to try it out - it will arrive this Friday. I know it's good but as it's heavy, I want to see how I can cope with it as my preferred photography is wildlife and birds in flight. My current lens is 18-300 which is ok but not good for my type of shots. My question is, with all the hype about mirrorless, should I have gone for a mirrorless instead of the D7500? I think I did read somewhere that lenses for mirrorless are also heavy.....

A few months ago I upgraded my Nikon 3300 to the D7500 which I'm very happy with. I've also rented the Tamron 150-600 G2 to try it out - it will arrive this Friday. I know it's good but as it's heavy, I want to see how I can cope with it as my preferred photography is wildlife and birds in flight. My current lens is 18-300 which is ok but not good for my type of shots. My question is, with all the hype about mirrorless, should I have gone for a mirrorless instead of the D7500? I think I did read somewhere that lenses for mirrorless are also heavy.....

A few months ago I upgraded my Nikon 3300 to the D7500 which I'm very happy with. I've also rented the Tamron 150-600 G2 to try it out - it will arrive this Friday. I know it's good but as it's heavy, I want to see how I can cope with it as my preferred photography is wildlife and birds in flight. My current lens is 18-300 which is ok but not good for my type of shots. My question is, with all the hype about mirrorless, should I have gone for a mirrorless instead of the D7500? I think I did read somewhere that lenses for mirrorless are also heavy.....

There are THREE mirrorless interchangeable lens camera formats:

Full frame (FX)
APS-C (DX)
Micro 4/3

Only Micro 4/3 systems save truly SIGNIFICANT weight, because their NATIVE (not adapted!) lenses are 50% to 75% lighter than full frame lenses of the same coverage (angles of view) and maximum aperture.

If you drank the cool aid about buying full frame lenses for your APS-C (DX) camera because “some day”, you might want to “move up” to a full frame body, then migrating to mirrorless won’t save noticeably significant weight.

If you bought all DX lenses for your DX Nikon, you saved some weight, maybe 25% to 30%. But those lenses won’t be anywhere near optimal when used on Nikon full frame mirrorless.

If you sell your existing gear and switch to A Panasonic Lumix G9 or GH5, or an Olympus OM-D E-M1 Mark II, and buy any mix of Olympus, Panasonic, Panasonic Leica, or other brands of native M4/3 glass, you’ll easily save half to 2/3 of your current system weight.

You have to decide how important weight saving is, in your situation. If you decide to change, rent the interesting new gear and try it before spending money on a new system.

Personally, I switched from a Canon-Nikon mix to a Lumix GH4 a few years ago. It was the right decision for my needs. *Your* needs should dictate your path.

I would try the Panasonic Leica 100-400mm f/4-f/6.3 zoom (equivalent to a 200-800mm on full frame) on a Lumix G9 and an Olympus OM-D E-M1 Mark II. The lens performs better on the G9, due to the Dual IS II stabilization system, but these bodies are ergonomically very different and the menu layouts are VERY different. You’ll almost certainly fight one and love the other.

Thank you all for your comments. The majority of you said that my D7500 was a good choice for the type of photography I like to shoot....it has made me feel better to know that many of you are still sticking to your DSLRs. As to the lens, I'm waiting to receive the Tamron which will have for 3 days.
Got a couple of nature reserves near me so that's where I will be trying out this lens. If it's too heavy for me to carry around for hours will have to rethink.
However, I've got a sturdy Manfrotto tripod and monopod so if I do decide to buy this lens, I will consider a gimbal as well. Maybe in the future, I might go mirrorless because I'll have problems with weight (I'm not a spring chicken). I've also started going to the gym (never set foot in a gym in all my life) to try and get some muscles - surely this would help ha ha....Grace