Welcome to the addiction that is FT. Sorry about your back, most of us will probably be in the same situation at some point. Yes, as far as we know, must be targeted to get NLL offers which allow for multiples of the identical card (and SUB).

That’s the $64,000 Question*. Several theories but don’t think we know the answer to that question. There’s some evidence that having and using one AmEx MR earning Biz card leads to receiving these targeted NLL offers.

As a SUB fanatic I got an AmEx Blue Biz Plus about 6 years ago, at the time purely as a no-cost way to keep my MR alive, from various serial expensive MR cards and closing each account at 12.5 months. (Personal Plat then non-NLL Enhanced Biz Plat then Personal Gold then Ameriprise Plat (may not be available anymore) then non-NLL Biz Gold then Schwab Plat, etc.) I started receiving NLL offers about 2 years ago, about 4 years after opening the BBP, don’t know what the average or typical interval is.

It's all about the SUBs (for many of us). It’s a marathon, not a sprint.

(* “The $64,000 Question was an American game show broadcast in primetime on CBS-TV from 1955 to 1958, which became embroiled in the 1950s quiz show scandals. Contestants answered general knowledge questions, earning money which doubled as the questions became more difficult. The final question had a top prize of $64,000 (equivalent to $650,000 in 2021), hence the "$64,000 Question" in the show's title. … The phrase the $64,000 question is an idiom and is routinely used as a way of saying the most important question. It is derived from the fact that the ultimate question on the show was indeed, the $64,000 question.”)