Well the OP was also looking to maintain his FTP if possible.
I think it also depends on how much training (structured) the person did before whether or not a trainer is going to work.
I was 95KG+ in Feb’18, was able to drop down to around 85/86KG when I started with TR in Oct’18 and was 80KG in Feb’19 when following the plan.
My FTP went up in that time, of course this wasn’t only due to the trainer.
Looking after my nutrition, rest and doing strength training helped.
But a one point my weight loss plateaued which isn’t a surprise, I think when that happens it’s time to change things up a bit if someone wants to break through the plateau.
I love mountain biking, but a trainer is more time efficient for me plus gets me in good base shape to enjoy the times I am on the mountain bike more.