A build high up in the sky will be fine(2000m - 4000m), your viewer shouldn't load anything from the ground unless you cam down.
The concerns for a Mainland build are being connected to 8 sims, all of those neighbours and all of their unsightly builds. And you probably wouldn't use a 'sim surround' as it'll extend into neighbouring sims and onto their land. Physically it's not there and they could just derender it, but that depends on the virtual neighbours you have.
Depending on the size of your parcel you could use a sphere/dome. Rez a megaprim and make your own enclosure, or buy something like Turnip's Skydome for 1000L, they go up to 180mx180m, large enough for a 1/4 plot.
Tips for building in the sky! Know the size of your parcel. Ctrl+Alt+Shift+P turns on parcel lines, build a box within those borders for your platform. When it fits perfectly, in your Edit tab, the "Position" X and Y give the centre of your plot. These are the coordinates your dome will sit on. When you OCD on numbers like I do, then alignment is important
Send the platform up by changing Position Z to 2000m. Then easily transport yourself up with the go-to-height chat command:
flr -(floor)will send you back to the ground level.