What makes sense for one person may be totally wrong for someone else. That said, here is what I recommend -
I would not sink money into your old bike. It is outdated and limiting at this point. Bike standards are changing so fast right now that what you buy today may not be usable on a new bike. You may find it hard to find parts for the old bike already. If you can get 'upgrades' on closeout or other wise low cost - it might make sense for you. But be careful how much you spend on that old a bike.
I don't find a GPS very useful for navigating trails. Even a 'high end' GPS does not have trail maps in it like they do for roads. So even if I load a GPX file into my GPS, I have to go past a turn before it tells me I am off course. And it CONSTANTLY tells me I am off course even when I am on course (GPS error? trail changes? Aliens?). What a GPS does best is record your ride so you can look at it after the ride, load it to Strava, etc. Maybe the technology has improved to where a GPS can communicate with Trailforks or MTB Project and I don't know about it. But my experience is that GPS units don't help much with trail navigation.
Phone navigation of trails works much better than my GPS unit. But I keep my phone put away so it does not become a victim of a crash. To use the phone for trail navigation I have to stop, take off my Camelbak, dig out the phone, dry the sweat off my hands, etc etc etc before I can get navigation info. All of that and I still find it more useful than my Garmin 800 GPS. Your mileage may vary.
So what is my recommendation? The GPS unit WILL transfer to a new bike. If you want a new upgrade now - that is where I would spend my money. Just recognize the limitations. And I would be looking for a new / used bike in the near future. I would probably save my money to put into the new bike.