I would suggest concentrating on the fact that your body recovered after your first DC. So it is capable of possibly doing that again with a second DC.
However a word of caution. It is MUCH harder coping with SPD when pg when you already have a small baby/toddler to look after as well. It is not the same as the first time round at all. You can't just take yourself off to rest when you need to and you may struggle to look after your DC1 in terms of bending, lifting etc, just doing everyday things. Ideally you will need loads of support from family and friends.
It is quite likely that you will get SPD again in a second pg probably much earlier on, because you are already 'wired' to get it IYSWIM. It is a myth that SPD is all due to 'hormones', if it were then EVERY pg woman would get it, although the changes in your hormones do play a part.
It is of course a totally personal decision for you and your DH and no-one else can tell you what you should do; only you know how you felt during your first pg, but it sounds like it was as severe as you can get and you really suffered - if it were the same again how would you manage looking after DC1 as well?
I'm not sure waiting longer would make that much difference in whether you get SPD again, but obviously gives your body longer to recover from your first pg.
I too had severe SPD with DC1 and went onto have DC2, becoming pg 18mths later but my age was a factor in getting pg again so soon (I was approaching 40). My desire to have a second DC was so overwhelming I felt it was worth the 'risk'. However I started to suffer with SPD more or less straight away and it was a very very difficult time.
I found that the NHS knows very little about this very debilitating condition. If you're determined to go ahead I would strongly suggest getting an appointment with an osteopath straightaway and to get them to help you before, during and after your pg. I really wish I had done that. Unfortunately the nhs physios will only offer crutches and a support belt without really trying to rectify the problems in your pelvis, which a good osteopath can do, even when you are pg. But you will need to find out if they specialise in pelvic instability first, as not all do. I have some good contacts if you would like to pm me.
If I had my time again, I would have got help from a specialist osteopath all the way through. It is only now when I am still disabled from this condition that I am beginning to find the right kind of help,and after a lot of research. I don't want to scare you about how I am now, because everyone is different. But I now have two wonderful children and a very very supportive DH, but it's not without a lot of stress and strain for both of us still.
Hope this helps you a bit. If you've got any specific questions please ask and I will try and help. Good luck.