Since OP is headed in a 'buntu direction, may I suggest trying linuxliteos and compare it to lubuntu? I enjoyed it when was on a kick in that subsection of linux. Or how about easyos?
Have fun!

Got a Dell Inspiron 1501 with an SSD which makes all the difference. Go for the hardware upgrade which will open up your options nicely IMHO.

I have a Dell 1525... on-chip Intel graphics...
Running into the 'flip_done timed out' issue which results in very slow boot/performance/shutdown.. :-/
I understand that this is due to to the Linux kernel being later than 4.4

Any suggestions for a LTR distro that will run *out-of-box, pref with a basic DE such as XFCE or similar?

*am Linux noob, but I know I gotta get clued up before Win7 gets abandoned... no way am I going down the Win8/10 route!
Have tried PuppyLinux but couldn't get my head around it.
Have tried Lubuntu 18.04 XFCE... am making some progress.. :-)

that machine is also ~10 years old.
antix or MX Linux.
antix is usually a good bet for old and very old hardware, MX Linux should be very similar under the hood, but with an XFCE desktop on top.

I would also recommend AntiX. Not only is it very lightweight but it can gracefully handle quite old and eccentric hardware.