Doing a lactate ramp test will get you a good idea of where your current thresholds are. It can also give you a good idea of where to focus based on your goals and current weakpoints. Idk how long you were away from training but big shifts in your lactate/power curve are possible after just a few weeks off from training.

Sea level to 7000ft is almost certainly going to increase your lactate levels at all but the lowest intensity, at least until your body acclimates. Less air=less aerobic contribution=more anaerobic contribution=more lactate produced.

4.0mmol is a nice number to know but may not be your LT2 or "maximum lactate steady state". Some guys can hold 5-6mmol steady and some guys only 2-3. Just depends on your physiology and training history. You can get a close guess at this on a ramp test looking at the inflection point.

IMO if you have a lactate meter there's not much point in doing FTP tests. You have a known power curve that you can use to define workouts and zones, that's miles better than just assuming you fit into the same zones as the average cyclist.

I'd worry less about lactate and be satisfied with the workout given your RPE correlated well. I definitely wouldn't try and target a specific lactate number unless you have historical data showing 3.5 is correct for you, as it is both highly personal and dependent on training status.

Absolute lactate values are very individual and even sport specific. My LT2 values on the bike are always around 4.5+mmol, so I’d expect around 4.0 for sweet spot. My LT2 on the run is ~3.5mmol and usually work on the 3.0 range for similar effort.

You really need to do a lactate ramp test (multiple over time) to really dial in your individual zones. It could be that FTP is set to high or just as likely that you are in the correct zone.