You don't have to use the IP address one if you already have strong reason to think it won't work.
I'm curious if you see a reduction in DNS spam with the other ones enabled as blacklist rules. Different devices sometimes react differently to each kind of blocking, from my own testing at least. The default blocking style is reply=nodata so you can try reply=nxdomain, reply=none, and reply=refused and see if any of those will make the fitbit give up.
Something else (I'm just now reading about) you can try that will almost assuredly reduce DNS traffic:
Add BLOCK_TTL=60 to your /etc/pihole/pihole-FTL.conf file.
You can put whatever you feel comfortable with as a number there. The default is '2' - raising it to 60 makes it a full minute.
This setting is supposed to apply a TTL (Time To Live) to the DNS responses for blocked domains. In this case, I'm recommending a TTL of 60 seconds. The upside with this in effect is that devices are supposed to wait a full minute before requesting a blocked DNS name again. The downside is that it'll take a full minute before any new whitelist rules come into effect.
You can bump this up to an hour or more if you want to really cut it down, but be warned that will also make new whitelist entries take that long to engage as well.