Tricky times OP, and difficult if you cannot come to a feeling that is 100% comfortable.
My own thoughts on reading your OP:
I would worry about money. Things will not get easier over the next few years, and with huge extra expense of a 7 seater car, bills, everything, ability to provide any holidays, groceries, needing two of everything for the babies etc etc. I would worry.
Plus the impact in the short and longer term on your earnings and your business.
5 to potentially support through Uni - eek!
Yours and DH's job security - OK? And your pensions? Because adding more child raising years can reduce the years in which you can catch up and save (ask me how I know!)
The practicalities would do my head in - meals for teenagers, meals for small children coming in from school, huge quantities of washing, queuing for the bathroom if you only have one...you have the teen years upcoming for your 2 eldest...
You're young - did you have your first at 19? I dunno - when will you and DH get a life to yourselves? I think young families are great but by the time any new babies leave home you will be 49. You might be craving some independent life by then. 40 yo men are notorious for wanting to spread their wings, take to their bikes, not be tied down to family. MN is full of it.
I would worry about impact on other children. By the time your eldest is starting GCSE years your youngest will be in school. The GCSE and A level years are quite intense, support and stability is important if they are to do their best. OK the baby years would be done, so that's good, but it's a lot of parenting needed - reading practice with twins, teens in the house..... Of course amongst the stats plenty do really well but the truth (established in many studies) is that children from big families do less well educationally than those in smaller.
I have a friend who has 5 kids - 4 are thriving, very bright and high achieving and as a family they manage to do a lot together, have a lot of fun and manage in a modest house with average income. BUT the one who has been diagnosed with ASD conditions is not thriving at all. And there isn't a lot of space, time, money or quiet to support their needs. (they get a lot of support from family, including financial, but the Mum hasn't been able to work since the youngest 2 were born)
However if you thrive on parenting and motherhood, love domestic activity, can cope with huge piles of washing with good cheer and energy, and if you and your DH feed each other's enthusiasm and love by raising children, then go ahead and enjoy raising a big bursting at the seams family!
You and your DH need to discuss what you each gain and lose from each scenario, and what you gain and lose as a couple and as a family...and ultimately what you can live with and what you cannot live with in your decision.
I really hope it works for the best which ever way you go.