the max current the BMS will need to handle will be minimal compared to a 14s system.
On the contrary, the current required would be much higher. On a 14s system to power 500w load, you'd only need 9.6A @ 52V. To power a 500W load on 1s, you would need 131A @ 3.8V!! Essentially, you would need 14 times the current to go with 1s compared to 14s.
The lower the voltage, the higher the amps. It's a law, and it can't be avoided or ignored.
Sure, you could have a
lot of wires hanging off this pack to power a
lot of boost converters, but that would be a nightmare to troubleshoot, increase chance of shorting something out, and wire management would be a pita.
The BMS is only be needed in the charge cycle
The bms needs to monitor not just charging, but also discharging. Because you want the bms to disconnect the battery before it gets too low.
My real interest is to see if I can find a higher current Buck converter
Once you go over about 30A for a boost converter, you are starting to get into drastic diminishing returns. The cost of the units go up drastically compared to the output current. 3.8V * 30A = is only 114W.
If this is a hobby build and are quite aware of the how
much this is going to cost you, by all means continue forward. But it doesn't seem like it when you state your first pack is 1000cells and plan to add more. You will quite literally be spending at least 5 times the money to build a 1s setup with nothing but boost converter(s). A decent BMS that can do 7s or 14s will be in the range of around $100-200USD. For just the boost converters you are looking at well over $300USD to be able to get a good amount of power available. And then there's the heavy gauge wiring that must be used with low voltage/high current.
I'm sure a lot other members here will agree, you really need to re-evaluate the build, materials, and more importantly cost and time of this build. Sure, it can be done. But at what cost, and compared to more practical ways? Can you fly from LA to NY in a bike-plane? Sure can. Is it practical? Nope.