Unfortunately you have -- very understandably -- been tripped up by the terminology, which is confusing.
The difference is between the letter "b" -- that's lower-case -- which stands for "bit"; and "B" -- upper-case -- which stands for "Byte", where a byte is 8 bits. Camera record speeds are generally in bits, and memory card speeds are generally given in bytes. You will see a lot of people online get this wrong.
So your camera does actually record at 120 Mb/s -- that's mega bits per second. Divide by 8, and we can see that that is 15 MB/s -- mega bytes per second. If you have an SD card with a v30 speed rating, that is 30 MB/s -- mega-bytes per second, which is easily enough.
BTW when you say "a reasonable SD card has a writing speed of half that value (60-70Mb/s)" -- I think you mean MB/s, but in any case you need to know that the speed numbers printed on the card are useless, misleading garbage. That number is the FASTEST possible speed that the card can ever go at, which is completely irrelevant for shooting video, where we care about the WORST-CASE speed. That's why you should always look at the "v" speed rating, which is actually designed for video.
I made a series of videos which try to explain all this, which may help: https://moonblink.info/MudLake/tech
The videos "Bit Rates for Video" and "Memory Cards for Video" might be relevant here.
