What is a healthy weight loss pace?

The further along this process I get, the more I think the ideal pace is the one that you feel you could maintain for the rest of your life. I still sometimes feel (mostly when I step on the scale and don’t see a drop) that this is going too slow. At those times, I ask myself “what would you change about what you’re doing, knowing you’ll need to do that long after the weight is gone”. More and more, I find that the answer is “nothing” and that helps me have confidence that this time I will be able to keep it off.
A few tricks I use to keep that mentality up:
-Track things in addition to weight so you can see progress when the scale is stalled and focus more on health than weight. With help from my Apple Watch I’m tracking number of steps, VO2 Max, average heart rate, average respiratory rate, and lots of other fun stuff. I track measurements and time to finish a mile and other health victories as well.
-Look at average weight loss, especially when you feel stalled. I still need to do that, and it reminds me that my average pace since I started is >1.5 lbs/week and my past 8 weeks is >1 lb/week. I’m good with that dropping to <0.5 lbs/week as I near my goal as long as I feel like I can live that lifestyle indefinitely.
-Be willing to change the things you aren’t loving even if it slows down your loss rate. Truth of the matter is that it’s the idea of having to do those things forever that has made me give up in the past, so they don’t really make it go faster.
Now for the textbook answer- 0.5-1% of your body weight is considered an acceptable and sustainable pace, with the percentage lowering as you near your goal weight.
Best wishes on your journey.

I'm not surprised your weight loss rate has slowed with a PT: working out harder = more water retention for muscle repair and/or less loss of muscle mass while losing weight.

I'm a huge fan of slower weight loss: less muscle loss, less hunger.

Generically, I usually suggest 0.5-1% of current weight per week as a max, with a bias toward the lower end of that. Someone severely obese could lose faster, but they should be under close medical supervision IMO for deficiencies or complications.

But you're not generic, you're an individual.

Like others said, it matters how much fat you have available to lose. It's smart to lose slower as one gets lighter. (Research suggests we can only metabolize a certain amount of stored body fat daily, and that is a loss amount per pound of stored fat.) As loss rate increases, risk goes up for losing unnecessarily much lean tissue alongside fat.

Nutrition matters, probably, too. Moderate loss with poor nutrition might risk more lean tissue loss than the same loss rate with excellent nutrition. I'd guess the same about exercise stimulus: More likely to lose more lean tissue when losing at X rate if not reminding our body we want to keep it.

Beyond that, assuming body weight itself is not an acute health risk, "best" weight loss rate will vary with athletic or fitness goals. Certainly someone strength training (for either strength or mass) is likely to get better results with those if losing slowly rather than fast. It's probably somewhat true for other athletic goals as well - training for races or whatever.

I think this probably doesn't apply to you (you didn't mention it), but I'd also vote for a slower loss rate if there are other substantial physical or psychological stresses in a person's life. (Examples: Stressful job, family crises or drama, health conditions, big ramp up in exercise, etc.) Stress from all sources is cumulative, and too much stress can have negative consequences on physical health as well as mood and such.

If you've been working with a trainer, but doing about the same or more exercise activity as before the trainer, plus doing the same amount/type of daily life activity (job and chores, etc.), and eating at the same calorie level but losing slower, I think one of two things is likely:

1. Water retention from the exercise. (8 weeks is a long time for that to mask any but slow fat loss, though, I think - based on experience. I've gotten to maybe 6 weeks, but I was losing at less than half a kilo a month (on purpose)).
2. Exercising too hard with the trainer (or overall), getting fatigued (perhaps subtly) to the point that it bleeds calorie burn out of the rest of your day. (Even fidgeting can burn up to low hundreds of calories daily, according to research. Would you notice if you fidgeted less? I wouldn't.)

If you're not calorie counting pretty meticulously, there's another option: You're eating more. Exercise tends to spike appetite, and if not counting, or eyeballing rather than weighing portion sizes, it can be easy for calories to creep up.

TL;DR: The "ideal" rate isn't universal, it's individual.