Partial antifreeze changes, pros & cons + can`t do the math :(

Any one in the USA done this? interested in a US perspective.
(hello from the UK)

Coolant system holds 6 litres. Header tank holds one and a half
Litres..(ethelene glycol at 50% as per manufactures spec)
I can suck out the header tank in seconds with a vaccuum fluid remover
and replace with fresh.
The engine bay is a nightmare for access, none from above to coolant
hoses (no drain plugs)
Even from underneath on ramps, lying on my back removing bottom hose
from rad , arm reaching up at full stretch into the engine bay, results
in an antifreeze shower down the arm, and only removes two and a half
litres.
To get more out from the block and heater hoses is even worse.
I wondered if anyone else just sucked out the header tank every 3
months or so instead?
Never had the pleasure to see under the hood of an American vehicle,
but hope you have more room in there , in general for access.
I was hopeless since shool in even simple math.... Ok.. even
arithmatic
But if I sucked out the header tank 3 times a year, can anyone tell me
how much percentage of old coolant I would be running with in the
system.
Any thoughts on this?
I`m just retired and trying to save on servicing (labour)costs, but
mainly trying to avoid a half day crawling under the car, soaking in
coolant.