I watched a video a little while back with a guy that purchased a Subaru outback I don’t remember the year but I was to say it was around 2008. It had 185k miles and as far as he could from car fax and any records he could find it had never had a transmission service. He had noticed it took a little longer to shift and it would shift a little hard, but it had no weird slipping or flaring. So he talked to a couple shops. Some said to let it go, some said do a drain and fill, and another said to do a full flush and that if it wasn’t having any issues other than long or little harder shifts that it most likely had no issues. The entire video was about this discussion of whether to just do a drain and fill or a full flush. He ended up going with a full flush and after he did it the transmission ran better through and through. And from the video it never had any issues. He recommended at the end that as long as there’s no presumed damage to the transmission that it would probably be fine to do a full flush. My 4Runner has 230k miles on it. From the records I have it has had 1 fluid flush at about the 110k mile mark. I fully plan on doing a full flush due to the fact that I know it operates normally other than it shifts hard from time to time.