in your case, as you say your child is physically aggressive, I would consider what would happen if he lashes out at baby/toddler/sibling as they grow up together.
also, never minimise the impact this will have on both children - your older child will lose time and attention. Your second child will also becasue there will be times you cannot be there for them.
what is your support network like? Do you have friends and family you can get help from now with your autistic child? Are there sibling carer support services in your area (they generally take referrals from age 8 of the carer child).
how supportive is your workplace? Are they flexible towards families.
tou have to consider everything you would ordinarily and the extra things. Like, are you having another child and hoping that some day they may take over with their older sibling if/when you are unable to - this cannot be relied upon.
Are you prepared if your second child is also as severely autistic - can you manage two potentially very different sets of need (speaking from experience).
ultimately, only you know what your heart is telling you. But you need to go into this with your eyes wide open of you might get a shock in a few years if things don’t happen as you plan/hope.