Spirited, I know you feel you are up against the (biological) clock, but really, your DP is 'untested' as a dad. If I were you, I would want to see how he was as a 'resident' father to your existing children, day in day out, before I thought about planning another baby with him - because being a weekend-only dad is a very different thing, and IMO much easier.
I can provide a bit of a cautionary tale... DS1 was 3.5 when I met DS2's dad. I got pregnant after only 5 months (pill failure, and I decided not to have an abortion) so it felt too soon for him to move in. I was very cautious about moving a 'new man' into DS1's life so soon. (And I owned my own home, while he rented, so I had security and independence). So he came to our house twice a week, and it worked well. I think it felt like a little 'holiday' for all of us, each time: DS1 got an exciting new visitor, I got some help, he got a break from his normal routine... :)
Unusually, after DS2 was born, we still did not live together - and our arrangement continued to work fairly well, though other people thought it was odd. And when I went back to work p/t, he did some of the childcare, so he was at ours 3.5 days per week on average. We had different parenting styles and habits, of course, but he was pretty hands-on, looking after DS2 and doing the school run with DS1, cooking meals and doing nappies and all...
When DS2 was almost 3, we decided it was time for him to move in. Almost immediately, we both felt the strain. He had been used to going off and having child-free time in which to work quietly, and I had been used to running the house my own way! Differences began to feel bigger and more problematic - I came home from work to find he'd forgotten to feed the kids because he'd been trying to work, for instance, not once but several times.

Then when DS1 hit puberty, he wasn't very good at dealing with it at all. He wasn't used to hormonal strops, and they made him angry. It started to feel like he was favouring DS2 (his blood child) over DS1. He lost his temper in a frightening way a couple of times, and wouldn't acknowledge he was in the wrong... There were other problems I won't go into here... We lasted together less than 18 months from the date he moved in. :(
Very, very sadly, after we split up, he saw less and less of DS1, though he continued to see DS2 regularly. It confirmed my fear that he didn't care for the two boys equally - and my poor DS1 became 'fatherless' again. :( :(
You might avoid all of this, of course. Your DP might rise to the challenge and be a marvellous full-time dad to all your children. I hope so. :) But the worry is, if you agree to try for a baby before he moves in, and then it doesn't work out, and he turns out to be best suited to being just a part-time dad, then you will be left in a very vulnerable position...