Sounds like you got many years of service from your old legacy controller that you currently have, and yet you still have second thoughts with getting the newer model from that company. If you get 12 or more years out of a new head unit, then I think thats a good upgrade.
Here are my thoughts on the APEX to look at this from a different view than you currently do.
I actually had their initial controller then upgraded to the legacy and rode that thing out till the saltwater eroded the head unit out. I sent it in to get fixed and they had a program where if they couldn't fix it...then they would give you discount replacement price. Unfortunately, at that time they no longer made them. So I just had to buy a new 2016 head unit on sale. Not sure they offer those now.
Lets face it. That is great when a company lets legacy users upgrade their product by buying just a head unit. you can keep all your old stuff and its completely compatible. Apex has been around for long time and they stand by their products and it seems like they try to keep all generation of users on the same playing field as far as interoperability and functionality. Its kind of "If its not broken, then dont fix it". They have come out with lots of new stuff for the APEX since 2016, and everything always hooks up to their head unit. No need to go buy a "2022 Model" to hook up their new "XYZ"., and have to shell out dollars and dollars just for the latest and greatest attachment.
Well anyway. Thats my take. You got me thinking about my Energy bars on my display. They are older than your head unit and they are still going strong.
As some one said. The newest one out is probably the CoralVue hydros in 2019. You may want to give that a try, if you just like to have the shinny new thing on the block as far as a head unit. I think it will be more of an investment as you will have to scrap some of your old equipment as I'm sure it will not be compatible outside of a probe I would think. If you do go that route. It would be interesting to see a review of the legacy Apex to the CoralVue hydros in your eyes to see which one you like the best. The 12 year work horse or the new one. Keeps us updated on the decision.
I really like my old apex. It's not that I'm always looking for the latest and greatest, but I do feel that technology is always advancing. It's a big expense and I'd rather not buy new at the end of a development cycle.
As you point out, the apex I have has served me very well. I think the first option will be to try to repair that unit. There are both software and hardware fixes that I can attempt, and I'm not afraid to break out the soldering iron. If that works, then I will probably just continue with my current "classic" unit until the next major issue, or until there is a new version worth diving in for. If I'm not able to fix it, I will be look for a used base unit. Like you said, I can still use all of my other apex hardware with the new base unit and enjoy many of the benefits of the current generation controller (like native wifi!).