Hello and welcome!
I'm worried that if we leave him on too low a level for too long other complications may arise.
It is a struggle, balancing these concerns! I see why your vet wanted to reduce, however-- you've hit some very low numbers (40's on an AT will make you sit up and take notice!), and while being in high numbers can do damage over a long period of time, it's the hypos that can kill quickly. When you say that the libre readings were "inconsistent", does that mean he was going really low, then really high? That's a really common pattern, usually indicating that the cat is on too much insulin, so I can see where your vet is coming from here.
To answer your basic questions, yes, it definitely does happen that cat's insulin needs change over time, so that what was once the perfect dose no longer works well. It can take time and patience to figure out the new dose-- the testing you are doing is going to help a lot in this!
Couple more quick observations to throw in:
1. Changing by 1U at a time risks missing the perfect dose (around here we change by 0.25U), so it's possible that all the changes your vet was making while Benny was on the Libre were just too much too fast to figure out a good dose for him. Additionally, looking at the dat you do have right now, it's possible that 4.0U is too much but 3.0U is too little.
However:
2. I'm not sure we can really evaluate the the 3.0U dose properly yet. The mid-cycle tests you've been getting are great, but we also need to see where he's starting from i.e. the pre-shot tests. Would it be possible for you to start getting those numbers again?
3. If you're thinking of switching to a long-lasting insulin, you might want to consider Levemir as well as Lantus if your vet is willing to prescribe it. They are very similar in their actions, but at higher doses the Levemir formulation has less of a sting. I don't know that Benny will end up being a high-dose cat in the end, but it's a possibility given that he was on 5U and doing well for a long time.
Lastly, a quick note about the AlphaTrak: it's a great meter, but the strips tend to be really pricey. A lot of us here switched to human meters for that reason, to the point where all of our dosing methods basically assume human meter readings. We can help you figure all this out no matter which type of meter you prefer to use, just throwing that out there as another option if it might work better for you!