Regarding a cross-platform version of LINQPad, I'm actively working towards supporting MacOS. I can't give a timeframe right now, but it might be sooner than later if XPF turns out to be a viable option.
That's correct - we aim to ship a new major version every ~2 years, which for now aligns with every LTS. So LINQPad 8 will support .NET 9 (in fact, it already works with the .NET 9 previews.)