To try and put some things in perspective: I was an early Z series adopter and got my Z7 in early October 2018. In 2019 I went on a 5 month, 24,000 mile road trip and shot over 40,000 frames with the Z7, all long before the major 3.40 firmware update was released. The camera certainly had some limitations with the earlier firmware versions along with the naysayers deriding the single XQD card slot. Nevertheless I was more than delighted with the Z7 performance for my use case on that trip, including a few successful bif shots. In addition I did not, and have not subsequently, had a single problem having only one card slot with my XQD cards (all Sony 120 GB) which I still regularly use today.
Then along came the Z7ii (and Z6ii, of course) with two Expeed 6 processors, a second card slot, faster stills frame rate, eye tracking in both stills and video, double the buffer, double the 4k video frame rate, etc., etc, along with the everything the original Z7 currently offers with the latest firmware installed.
Now consider that the Z9 has two of the next generation Expeed 7 processors and whole new set of features that did not exist on the earlier Z models. Note also hat the Expeed 7 has 10x the performance of the Expeed 6.
In my opinion that last sentence clearly indicates that no amount of firmware updates will bring the Z6ii/Z7ii cameras anywhere close to the performance of the Z9, including autofocus performance that requires processor "horsepower" that is not available in the Expeed 6. Now hopefully over time a few more features of the Z9 software will trickle down to the Z6ii/Z7 series but they will be limited to the constraints of the processor generation in the latter and I think it's therefore unrealistic to expect much more.
Facts: The original Z6/Z7 firmware was very much version 1.00 (almost a late version beta release in some respects) so it needed a lot of work to optimize it for the capabilities the hardware had to offer. Conversely the Z6ii/Z7ii's had well baked firmware (migrated from the Nikon's Z6/Z7 learning curve) from the start so were already a relatively mature product release. Similarly the original firmware on the Z9 was very much version 1.00 and needed several updates to bring it to a reasonable match with the hardware. Would I like 30 or 40 frames/second RAW on my Z9 and/or precapture for even 0.5 seconds at 15 or 20 frames/second RAW on my Z9? Of course I would!! Do the dual Expeed 7's in the Z9 have this capability? Realistically only Nikon knows the answer and we, it's customers, will only find out through future firmware updates, if any.
For what it is worth: I've fortunately had my Z9 for about eight months but I still regularly use my Z7 for work that is suited to its capabilities and it still does a really fine job.
Sorry for the long rant!