So to summarise, I think my options are either:
- Go with the rx10iv
- Get the OM_1 at a discount or used
- Something else? If there are any Cannon/Nikon/Fujifilm mirrorless options that anyone would recommend, then please let me know!
I would say go with:
RX10 IV (25x zoom) if you care more about zoom range
FZ2000/FZ2500 (20x zoom) if you care more about built-in ND filter.
I'd be tempted to go with the Sony rx10iv because of the weather sealing alone. I don't feel that the Lumix's are weather sealed. I could always use a physical ND filter to get the results I wanted as well, right? If anyone has any recommendations, I would be very grateful for your responses!
You currently have 30x zoom, which you might miss once its gone more than you think.
This is true however (I don't really use my bridge camera that frequently anymore) I feel I have taken far more photographs that are wider rather than are closer, if anything I feel like I would want to have a focal length that is marginally wider than much closer. I think I would like a 300mm maximum, or something on those lines?
As an example, when I was on holiday in Wales and I took my camera I only zoomed in my camera marginally taking a photograph of this mountain. I suppose if you were on holiday you would generally only take photographs of landscapes rather than zooming it in too much, so this is a bit of a moot point, but I haven't used the zoom functions as much I have previously.

Detail of mountain in Wales, taken with Sony hx200v
This is also because the zoom isn't as high quality as I would like and it does tend to deteriorate in quality, especially in low light. This will presumably be corrected by the rx10iv but it's something to consider. You can see this in practice, in this photograph that I took at the Commonwealth Games in Birmingham.

Athletes at Birmingham 2022 Commonwealth Games, taken on a Sony hx200v
Going from 1/2.3" to 1" you will see a big improvement in low light performance, but also you will notice that the depth of field is shallower, i.e. a scene that was totally in focus on your HX200V, could now only be partly in focus.
I wouldn't recommend going bigger than m4/3 since the "all-in-one" lens solutions have poor optical performance or they are really huge. As well, each boost in sensor size = shallower depth of field.
This is a good point. I will probably not go bigger that m4/3 in which case. Admittedly the Sigma 16 - 300 is only 600 grams which isn't that heavy, although it entirely depends on which body you want to use.
Unrelated, but I am also considering a DLSR, assuming they are still recommended? I don't know if this is still the case considering so many people are switching or have already switched to mirrorless systems.