Just set the fake google bot rule to log only, you can do this with yandex bing baidu etc as well. Do not give google asn a bypass all waf. Also, if this was a major issue others deindexed as well and much more complaints would be logged, i suspect this blocking isnt the issue, just think.
Check google console indexing report, how many urls are 403 as this is clearly stated, what are the google bot logs in settings crawler stats you did not post these so a few things to check.
if you post these since if its crawling and http status 200 from googlebot you been spanked sadly by google with an algorythm update. Blocking several googlebot i.p addresses out of thousands i would be surprised if this was the case.
Keen to see the indexing section report of 403 and crawler page stats. Curious to see