That is exactly what system I have to work on!
Cast iron oil boiler with 28mm gravities to cylinder and 28mm pumped (pump is also on return pipe) to rads. No controls.
The feed pipe joins in at tee at return at cylinder.
Your new pipe up arrangement would be nearly what I would ideally do, but difficult on this house. I might risk trying to seal the system - but at a low charge.
As to Vee’s point about sealing the system using gravity feed from f&e tank and non return valve, - I have came across systems done that way, some of which are fed from the cwt cold supply.
Personally I don’t like it because risk of contaminating cwt, also disadvantage that the sealed system will keep being topped up in an event of a bad leak, like nail through a pipe. Although constant top up has also advantages.