I own the Tascam 122 mk3, Teac V-8030s, Sony KA-3ES, Pioneer CT-43, a couple of Nak DR-10s and a vintage Pioneer from the early 80's.
My opinion is if you are transferring to digital the Tascam may be the best and most uncolored of all. Of course over the years any deck can be out of spec so people may have different experiences. The Tascam has the most clarity of anything I’ve heard. Its strength in that area led me to the Teac V-8030s and it doesn’t disappoint. It’s more of a romantic sound compared to the Tascam but equal in clarity and is probably my overall favorite. The Sony is great and was my reference for awhile but I’d rate it behind the Teac and Tascam, overall. The vintage Pioneer is pretty great, in fact. Once my tech serviced it, I couldn't believe the sound quality.
The Nak DR-10s are pedestrian and short of the performance of the others. Not as much extension, saturation, clarity, etc. But the DR-10 is easy to change belts on when needed and is the newest of all of them. Both my DR-10s sound the same so I’m confident saying that’s just the way they are. I plan to sell both and while they’re probably better than anything made and sold today, they’re behind the others as I hear it.
If your Tascam is working well, it might be the best option for doing your transfers. You can most likely still find someone to service them, too.