Here's my experiences in a nutshell moving from LG P500 with Froyo (old baseband) to Hardlight's LineageOS 11.0:
I made progress in three leaps. The first leap was getting rooted and getting Recovery installed.
1) Install Rom Manager from Google Play.
2) Use adb to sideload One-Click-Root, or copy it to the SD card and used a file manager to install it.
3) Run one-click-root and get Superuser
Now the benefit of experience. While you're here, open a shell from Superuser and copy the following files from /system/lib to /sdcard:
libgstk_exp.so, libpbmlib.so, and libwms.so. These files will come in handy later if you want your mobile calling/data/sms features to work.
4) Run Rom Manager and install ClockworkMod Recovery. Ignore everything it says about TWRP.
5) Select Boot to Recovery. Congrats you are in ClockworkMod
The second leap was trying several ROMs and finding out what's good and bad about them.
I tried several ROMs. I tried a gingerbread one first and found that like with Froyo I couldn't use Google Play, but I could sideload some gingerbread apps.
Next I tried CM10 and CM11. Various ROMs that had youtube demos, all looking a lot smoother than they were in reality, probably because the developers pegged the CPUs at a high overclock for the purposes of the demo, and didn't demo any real apps... just navigation and settings. The reality (broken Play Store, glitches in Status, terrible slowdowns) was very disappointing and I returned to Froyo for 2 months, with an occasional attempt at a different ROM that promised to provide a better experience.
I had seen LineageOS early on and I didn't want to give it a try because it seemed like a LOT of work to set it up, and running off the SD card seemed like a pretty bad idea from a stability point of view, not to mention that using the phone in USB-Storage mode was a favourite feature and that was obviously going to be impossible with the OS on the SD card. But over time I saw that all the ROMs were basically unusable without moving huge amounts of the software onto the SD card, so I wasn't going to be using USB-Storage mode regardless. So eventually I took the plunge, earlier today.
LineageOS 11.0 built by Hardlight seems to have solved almost every issue I had with the other ROMs. The wakelock is still there but the rate it burns the battery down at isn't quite so terrible. The phone doesn't double as a hand-warmer. Standby uses maybe a bit over 1% per hour, (vs 1% every 6 hours on Froyo with wifi active and gmail syncing) The camera is improved but slower, and I can run some apps I couldn't run on Gingerbread. But more importantly, it's been completely stable.
The third leap then, was installing Lineage. It's not as hard as it sounds.
First, you need TWRP. You can flash that from ClockWorkMod. Just download the 2.7.1 MULTI that Hardlight provided and put it on the SD card then fire up CWM and flash it. Job's done. The installation instructions say you need a version from 2016 or better, but the 2.7.1 worked fine.
Second, and before you flash LineageOS, back up your phone using TWRP, and then copy that backup off the SD card onto your computer. That way you can roll back to exactly how it is now if for some reason you're not happy. You'll be happy, trust me, but you'll be even happier having that security blanket.
Third, format the SD card and create the 4 partitions. I used a 32G Kingston UHS1 card that I got for $20. I made a 12G first FAT partition, then a 12G EXT4 partition, then a 500M SWAP partition, and lastly I gave the remainder (6.7G I think) to the F2FS Data partition.
After that I copied the latest LineageOS ROM onto the SD Card, along with the 3 libs, then flashed that and rebooted.
Wow it was seriously 10-20x faster to install than many of the other ROMs, and it booted in a tiny fraction of the time as well. It booted almost as fast as stock.
After setup I went straight to Settings->About Phone->Build Number, tapped 7 times to enable the Developer Menu, then I opened that and enabled Developer Mode and USB Debugging.
Then I went to the Terminal Emulator, 'su -' and copied the 3 libs from /storage/sdcard0 to /system/lib, and my mobile carrier came online.
The next part of my journey is still ahead: installing gapps, microG, and seeing what apps are available, stable, and have usable performance on this dear old indestructible plastic brick.
P.S. I figured out how to install the latest TWRP from the downloads link
Download both the latest, and twrp-3.0.2-0-20161230-OS2SD-p500.zip. Flash the .zip one and reboot to recovery, then click [install] and [image] to install the newest one. This was really confusing. Official TWRP zips lack the image file, and the latest here lacks the zip wrapper, lol... I'm sure there's instructions somewhere but they evaded my keen googling skills.