For 12 word BIP39 on average every 16th try will have a valid checksum. If I got it correctly there are only 2 dates 1900-2021, so the complexity is around 365.24²*121²/16 = 2^26.9 PBKDF2. Single address derivation (the usual non-hardened) is about 10 times faster than PBKDF2. Generating all the master keys would take about 1-2 minutes on 4xV100 (amazon p3.8xlarge), but to develop and test it would cost much more time.

Not worth it.

Let's look at the "hardest" 12 word "encryption". If only valid dates are supplied (i.e. no 37th day of 185th month), then the complexity is 365.24³*2048³/16 = 2^54.5 PBKDF2. Going through all combinations would take ~461 years on 4xV100.

Of course this scheme has an enormous weakness - since the dates are to be easy remembered, then the range would be significantly smaller. For example 3 dates in interval 1900-2021 give complexity 2^42.3, or about 35 days on 4xV100. Inserting a memorable date from the past doesn't help either.

It looks more like security through obscurity.