If there is any way you can try any 210mm I would do so. I bought a ssxl for 7x17 and found it too wide for my vision. I sold it to someone with a 12x20, making it a wider perspective. He says he loves it.
There are those who have that lens and do not need a center filter. It apparently depends on the subject. Jim at Midwest Photo (Columbus, OH - mpex.com) said to try a few shots before you buy the filter and then decide. I didn't get that far.
If you go to the LF Forum and search on "12x20 lens" there are several threads that might interest you. For that matter since you are just starting, there are many threads that come up when you just search on “12x20”.
Hope some of this helps.