I’m linking off my existing downstairs 6a circuit.that's not quite how it works to derate the circuit you should make the mcb smaller, or the cable bigger,
what size did you say was the cable you have is ?
the total length of the circuit matters and there is a very long list of things that we need to take into account when designing a circuit ,
Taking 1meter 1mm t&e link from existing switch to new switch to control new exterior lights for maintenance.
From new switch 4meters 1mm 3core&E to photo cell
From photo cell 4meters 1mm t&e to new light
From new light 4meters 1mm t&e to 2nd light
From 2nd light 15meters 1.5 t&e to garage light
From garage light 2meters 1mm t&e to 4th light
From 4th light 2meters 1mm t&e to 5th light
Cable going to Solar cell & new light & 2nd light will be behind wall insulation. Cable runs after that are not in insulation