I was tandeming at the start of one of my pregnancies but I miscarried at 9 weeks. ODS was a couple months shy of 5 and DD had recently turned 3. I used that opportunity to make weaning easier since I knew I didn't want to triandum nurse and have rough pregnancies/deliveries. I felt my need to wean ODS was greater than his need to nurse. In my other times pg, both nurslings happily nursed. I've been pg 5 times while nursing and miscarried 3 of those, so only two were full-term. However, all 5 times, my supply completely disappears at a mere 4-5 weeks pg. And both nurslings continued to nurse. (Actually, I now remember I was tandeming at the start of another pg that I lost- but that loss was at 7 weeks. ODS was about 4.5 years old then and DD 2.5).
Since you are interested in cutting down, I'd just figure it out as you go. You're not pg yet, so there's still time. If it takes 3 months to get pg - or you don't start ttc right away - and then 9 months of pg added in, you're now talking about your child being a full year older. And that's only if you get pg quickly. It's impossible to know what your child's needs will exactly be like a year from now. So I'd ttc if you're ready and figure out what to do as you go along. (However, keep in mind that 30% of nurslings will wean when mum is pg. And though you're fine with that with your older nursling, make sure you're comfortable with that possibility when it comes to your younger nursling).