Here you go...something like this will be a serious upgrade from a 388...and it's selling for $2.5k:
Otari MX-70 Professional Tape Recorder (1989) reel to reel recorder NICE | eBay
Yes that would surely be an upgrade for sure, but, again. You'd still need:
1) A mixer
2) More expensive tape
And, again, this listing doesn't specify that this unit is fully functional. It doesn't mention anything about functionality at all, actually. All it says is that it's in 'very good condition." I don't know about you, but that doesn't mean much to me at all at this point when it comes to ATRs. A lot of people selling these things simply turn it on and, if they see lights, they say it "works great!"
I'm not implying that you would, but I certainly wouldn't get in my car to go pick that thing up without extensive talks with the seller and video demonstrating full functionality. Not for something I'm going to drop $2,500 on.
I certainly wouldn't pay $2000 for a 388 (unless money were no option and I really wanted one), but that's the way supply and demand works. Back in the 90s and early 2000s, you could probably get one for $300. You could also probably pick up a Silvertone 1484 or 1485 amp and cabinet for $100 or something, if not for free. I mean they were Sears brands, right?
But then Jack White came along and used Silvertones, and all of the sudden, they're "legitimate" amps, fetching the same as vintage Fenders sometimes. The Black Keys (and a few others) used the 388, and all of the sudden it became associated with "that sound."
The funny thing to me is that, the quality of the 388 is really quite adequate, IMHO. By that I simply mean that its quality---assuming good recording practices, etc.---is not going to get in the way of the song to me. If the song/performances/source sounds/engineering practices/etc. are good, then I think you could record hits on it all day long.
Take this song, for example:
SISTERS - Back 2 U [388 Sessions] - YouTube
Now, this song doesn't shake my world or anything, but I think it's a pretty cool tune, and it's performed well. The recording doesn't sound "shitty" or anything to me. If someone told me they recorded it on a DAW, I'd believe them in a second. I wouldn't even describe it as "lo-fi." In fact, it sounds a lot "higher quality" than
many of the recordings I hear in mp3 clinic here that were recorded on DAWs. This is my point: that if you use pro engineers, pro mics, pro instruments, and have great performances, etc., then I think the 388 would do a fine job capturing the music more than adequately enough to showcase the song.
That's just my opinion, of course.
Anyway, I don't want to get too far away from the OP's original question. I think, as has been pointed out, you need to really get to the heart of why you're not really recording anymore.
I don't think I'll ever sell my 388---as long as it's working---because working with the machine itself gives me joy. I did go through something similar to your scenario though a while back.
I had a Tascam 246 that I loved. It was especially awesome to see it next to the 388, because they looked like brothers. However, when I accidentally saw a 246 listed for $1K a few years ago, I thought the seller was smoking crack. After doing a "Sold items" search and confirming that they were actually going for that, I decided---after much debating---that I couldn't pass that up. I'd bought my 246 for $200 and actually got a $100 refund because it wasn't working when I got it.
And this goes along with why I don't trust anyone who just says "in great shape" when it comes to ATRs. This 246 was described as "fully functional! Everything tested!" When I got it, I couldn't even get a tape in to start testing it because the belt had turned to tar, basically, and it wasn't even on the pulley. Anyway, I digress.
The point is that I had to think about what was really important to me. I knew I wanted a cassette 4-track, because I love having them for certain songs, but I also knew I could sell the 246 and replace it with another model and still have tons of money to buy other things I wanted. So that's what I did. I ended up with a Tascam Porta Two HS, which is perfectly fine for my needs (still has the cool vintage VU meters, too, which I love), and I also got a bunch of other stuff that I wanted.