Ok must have missed the first floor bit. But regardless your pipes are not ideal for thermo syphon anyway, need large bore.
As much as I say and agree gate valve are carp the 1s on you pump should be good enough to remove the body of the pump. As Grundfos (colour gives it away) keep the same body size for most normal sized heating pumps it’s also an easy swap.
I won’t comment on why you have so much pipe size reduction cos I am not a pro, but I wouldn’t do it.
Even though we can’t get them here if I need to get into our pump I have ball valves to isolate and only lose about half a litre, and bleeding out after is simple.
Did I mention I wasn’t a plumberer but I have a welding helmet, auto 1 it is too
J