Hi Chris, I'm still waiting as well.. submitted 751 since Nov 2019.. got an update on Oct 2022 stating that they used the biometrics. nothing since then.. like you we did everything from submitting request online, to asking help from senators, to seeking a lawyer's advice.. nothing worked.. all they do is ask you why don't you apply for citizenship when you already can? And to wait... The latest I-551 stamp I got is on a i-94 paper.. which is another joke.. Our case was originally sent to NBC then transferred to local office and now it's just sitting there building spider webs.. I doubt they are doing anything about it at all..
I'm tempted to do the mandamus but we don't have 5-10k sitting around just for this..
sorry that this won't be much of help.. just me venting as well..
if you're looking to travel, the i551 stamp will get you back in the country.. I've used them a few times traveling now..