The key thing is: what can you buy second hand at your destination. Not just "what is available", but "what are you actually willing to buy second hand" and "what about the other people involved". It's all very well saying "I will buy a second hand couch" but if your mother disowns you for throwing out the couch she gave you, or your partner refuses to buy anything contaminated by other people's posteriors, you've just traded a second hand couch for a whole heap of problems. Replacing it with a new one is environmentally a loss with very, very few exceptions (if you replace a solid gold or termite infested couch with a new Ikea one and recycle the gold/kill the termites, for example).
When we moved 1000km in Australia the basic rule we used was that anything where the cost of transport was more than 20% of the item cost was better left. We also shed most of our big furniture because that's easily and cheaply replaced[1], even for the few items we had paid for. The things we kept were our king size mattress (pricey), my workbench (difficult to replace) and a few items that the movers said they could fit in for no extra cost (we left a fair pile on the side of the road... most of it gone in a couple of hours).
What to take is in some ways less important that going through everything you own and thinking "do I need that" and "how hard is it to replace this". Now is a good time to downsize since you're going to have to go through everything anyway (unless you just blindly box everything up, which is a very bad idea... do you really want to pay to ship that box of old newspapers you forgot to throw out?) Ripping CDs, dumping that tatty collection of comics, "losing" the giant stuffed giraffe your first love won for you at a fair, whatever.
Plants are a much harder question. Small distances within an ecosystem, fine. And when the plant(s) are common at the destination and likely to survive both there and the journey, maybe. Shipping plants is hard, and some inevitably seem to die. There's questions about moving pathogens and pests around that have local and specific answers (someone introduced carnivorous earthworms from NZ to the UK and your moles are not happy about that, for example). So I count plants as "sentimental value or leave".
Another thing: years ago I arranged access to an A3 size scanner and scanned all my photo albums and other souvenirs then dumped the decaying paper remnants (photos fade!). Plus all my letters from high school and university (I'm old, ok). Those things are now much more accessible than they used to be (I sent copies of the photos to my parents, for example). That's a 20kg box of stuff I'll never move again. By comparison, my partner has a similar box that contains every laptop and phone she's ever owned. Most of them don't work and at least one of the batteries has leaked. But she will not let me recycle them because they're precious (even though she has copies of all the data on our current computers).
Yes, there is emotional pain in dumping stuff, but most people have stuff they only ever think about when they refuse to let it go. The physical stuff is easy... get someone else to help.
[1] in Australia people put large rubbish on the side of the road in the cities and there's a tradition of taking it if it's useful. So stuff like couches and shelving is easily obtained without paying for it. That makes the economics of paying to ship it around the country very easy: don't. Unfortunately the price of scrap copper is now high enough that people make a living driving round cutting the cords of electrical appliances that are put out, so it's now hard to get anything electrical this way. Which is environmentally a bad thing as well as being really annoying.
