I would stretch for the new AFP version even if good deals can be had on the old. The optics are far superior. Even if you have a DX camera I would get the FX version of the 70-300mm AFP as it has a better build including weather-sealing, 9 blades, metal mount and VR switch on the lens. Of course, it's twice the price and a little heavier.

I would also add that ON1 RAW has a better eraser for removing unsightly objects, e.g. bird poo or litter - they both do a decent job but CIP's eraser is often cumbersome to get right. I own an up-to-date copy of C1P, which is my goto program and also own ON1 RAW 2017, which I use for stitching and occasionally for its filters, though increasingly rarely as CIP's styles (expensive though as they are add-ons) now mostly do the (subtle) job I want.

About two years ago, I did extensive tests on several programs (LR, ON1, Luminar, Dxo Pro mainly) as I transitioned away from Aperture and the only other RAW convertor that is good as CIP is from DxO Pro but it lacks a few features I use in CIP and has no asset manager. mwsilvers is right, if you want a full-featured value program, ON1 is a great choice and the 2018 and 2019 editions add useful capabilities. But for best quality and tools combined with very decent asset management C1P is for me the only choice - but of course you pay: about €90 a year to keep it up to date after the initial purchase plus style packs - some of which are great, e.g. SPRING, and some less so to my liking.

Training is another important consideration as without it you will struggle to get the best out of any program. Here PhaseOne, the owner of C1P provide extensive tutorials and webinars on it, which are free and include guest contributions, which are often very useful as they highlight different ways of working. ON1 has many tutorials too but for serious training you have to pay.

I did a comparison between earlier versions of C1P and ON1 RAW - C1P won hands down, especially on detail and contrast. Feature-wise they are broadly comparable though ON1 has panorama stitching vs C1P has better colour management and tethering.

I've used the Breakthrough Night Sky filter to good effect. My goto lens for astro is a Nikon 20mm F1.8 (gives me about 25s) but the (manual) Rokinen 14mm F2.8 is frequently mentioned and used by astro photographers - they have a new AF version as well, not that you need it for astro.

On security, an alternative to gaffer tape on the lens cap is to use a third-party lens cap, e.g. from Breakthrough Photography, which are very good quality and have an alternative (no-name) logo.

I do take a mini-tripod - an RRS pocket pod but there are plenty of alternatives - it's very light and fits in a small bag, I can take group photos with me as well as low-light images and brackets - just need a raised surface. Agree with other posters on taking few lenses - my preferred combination is a 24-120 f4 and a 20mm f1.8 lens, thus my three section bag has a camera with lens, the other lens and a pocket pod. If I'm birding I might add a 300mm PF F4 and TC1.4 (in a larger bag) but not when with family.

I have the 300mm PF + TC1.4 and find the combination very sharp and lightweight - the TC is pretty much glued to the lens and I have auto-tuned the lens with and without the TC.

The disadvantage of the 300mm PF + TC1.4 combo is flexibility - it is galling if you want to zoom out and can only do so to 420mm before swapping out the TC. I tested a 200-500 in a shop but found it far heavier in comparison - perhaps with a monopod or gimbal it would work better - just more gear and weight to carry though.

It really depends on what you are shooting - do you frequently need to zoom in or out or not? I find for distant birds, e.g. at a migratory lake the 300mm PF+ TC1.4 combo works well and you can shoot for hours with little fatigue. When shooting birds of prey at a couple of festivals the combo worked less well for me as they swoop in and out. On a safari, I'm guessing the wildlife is mostly distant so less zooming; at a zoo animals can be both near and far so you need a zoom (or a second camera with a 70-200mm).

Just a thought - check your exposure compensation setting. If it is set to underexpose in combination with the highlight metering, this would seem to me the most rational explanation. Personally, I mostly only use highlight metering for theatre type events. Otherwise matrix or spot and the images you show would have all been well dealt with by matrix metering, even the first with a bit of sky as it is high and off-centre.

I also take a mini tripod when I travel - my choice is the RRS pocket pod and I use it quite a lot though you still need time to set it up and a convenient surface, which may be difficult when with a tour group. It can fit in a bag or pocket and provides stability when needed. My big tripod I just keep for car excursions and landscape photography.

I find the 24-120VR plus 18-35m works fine for most travel photography - you need the wide-angle in most European cities as they have so many narrow streets meaning many buildings can only be framed with a wide-angle. If you need a longer shot, then the D850 resolution gives you a lot of leeway to crop. Nevertheless, taking a more discreet camera than the D850 should be considered given the destination. A security tip for the D850 or any high-value camera is to use black tape over the Nikon logos and camera name and/or use non-Nikon lens caps; and of course, get rid of the Nikon strap, which is a horrible strap anyway.

As well as darkening the background, another suggestion is to clone or heal (out) the offending background items (most post-processing tools support such a technique) or (easiest) use the context-sensitive eraser in PhotoShop. If you are showing or posting your photos digitally there is nothing to stop you using non-standard,i.e. unconstrained framing of your crop, to find a composition that works. The default 2x3 or 4x5 crops are really only relevant if you plan to print your photos.

For security I also put black tape on top of logo and camera model number on camera and lens cap - as well as dump the logo strap, which is extremely uncomfortable anyway.

Black Rapid for sling strap - fixes to bottom of camera.
Matin Comfort Strap for neck strap.
Peak Design CUFF CF-2 for wrist strap.

In the cramped streets of Europe you will definitely need something wider. 27mm FF equivalent on a DX will be very restrictive. Consider the new 10-20mm DX - very lightweight and ideal for buildings, churches, etc. My 2c.

My recommended spec: 16GB RAM, SSD (possibly the most important for speed of loading previews) rather than HDD, good video card with 2GB VRAM and decent-sized monitor (14-15") with good resolution (so you can see your photos whilst editing). This means a higher-end laptop from whatever your favourite brand. Save money by not getting the highest speced CPU.