while I dont advise doing that, i'll say what I know, talk back is not preinstalled if you dont install the google apps pakige with in the rom

I dont advise installing any tipe of rom, because its kinda dangerse if you dont know what you're doing, and some roms are just vyruses, ETC

Meh. Android Pay is not as widely adopted, so losing that is not a huge price to pay. Paypal should work fine as well.
Also, there used to be a very efficient way to enable Talkback. Even if you get it installed through the full Google Apps package, settings are encrypted in the new versions of Android, so the old settings.db trick to enable Talkback does not work anymore. You are going to need sighted help to enable it.

You can use twrp through adb to install the rom through the command line no problem. You use this same method to install the gapps arm64 package. What you cannot do is modify the settings through the command line like you used to since the settings are encrypted. The accessibility shortcut has been broken since Cyanogen mod 11, and Lineage is basically Cyanogen where we left off before the team split up and Cyanogen became a commercial venture. The rom community as we know it has been blown to bits nearly as effectively as the organized jailbreak community has, since Cyanogen was probably the only well-oiled machine in the rom community. IT was a little too well-run though seeing as it got all those endorsements later on.
If you have another phone/tablet, Aira your way through enabling talkback. It should, in theory, take less than 5 minutes if you skip the initial google signin to enable Talkback, so the call can be free for you.

If you're referring to the Google Apps packages from opengapps.org, I'm pretty sure the Nano package doesn't include TalkBack. Find out what each package includes by clicking the links after the radio buttons for the distributions (I.e. Aroma, Super, Stock, etc.) Note that you should probably start with lower distributions and move up, because IIRC the higher-capacity packages include lists of apps prepended with "Everything in Stock, Super, etc. plus:" meaning you need to check the app lists for the smaller distributions for a full list of what is in the larger. Pretty sure you need at least Stock to run TalkBack, so that should save you some clicks.

Flashing a custom ROM is lots of work, though it is a good way to still use old hardware. Read about the openrecoveryscript feature of TWRP if you'd like to automate some actions. I'd still recommend sighted help, but you can at least write a script, push it to the device, and cut out the touchscreen interactions. My girlfriend isn't particularly tech-savvy but I can usually give her useful directions to navigate the TWRP process. But if that isn't an option for you, you can at least write a script and have your help tell you what happens, then iterate on that process until you're done. Eventually, once you've got a script that works, you can generally automate the process entirely by correctly placing your zips. One tip I've found is to always name zips update1.zip, update2.zip, etc. based on the order you'd like to flash them. They then appear ordered in the file manager, and it's generally easier to tell a sighted helper to click on update1.zip, etc. than "First click lineageos-16.1-pixel3.zip, then gapps-super-whatever.zip..." You can also use that tip to make your openrecoveryscripts more easily reusable. Just put your zips in the storage location in the correct order, then add/remove script lines as needed.

I like the idea of installing everything myself. So, would it work something like this?

Once TWRP is installed and I've rebooted into the recovery mode, I do something like this.
adb shell
twrp wipe system
twrp install update1.zip
twrp install update2.zip
twrp wipe data
twrp wipe cache
twrp wipe dalvik
twrp reboot

This assumes I've copied the files I want to flash to the internal storage first, and then wipe the data partition once everything is installed? I may  have the syntax wrong, I'm looking up these commands right now, so let me know if I have any errors. Then all I would need help with is turning on TalkBack from the Accessibility settings? Can I enable the volume key shortcut after that point to toggle TalkBack anywhere?

Oh nifty, didn't know you could shell straight into TWRP. Do you have to enable any settings, click any permission dialogs, etc.? Or is that available by default when recovery mode boots?

Um, yes. You can only ever do a full -w inside fastboot, and we aren't doing any of that here. Flashing a zip while wiping data and cache does the trick.
@Nolan: To my knowledge, no. Provided you are using vanilla adb drivers, you can shell straight into TWRP.

It's really easy to get a cheapish android phone to mess around with. I've used a samsung galaxy S7 belonging to a friend of mine to test roms on before.

Are we sure about @13's recommendation not to wipe the system? Does that not just install files over any already there? So maybe files that overwrite others work fine, but other cruft is left behind, and that's why you don't have enough space?

Although, that is a bit of a small partition, so he may very well be spot on. It's been years since I've done much with ROMs, and so much of what you find is anecdotal and practically ritual (I.e. my thing with update1.zip/update2.zip just so I don't have to spout out a bunch of odd filenames for my girlfriend to parse.)

I'd suggest wiping everything and trying again. If it doesn't work, drop to the smaller package and manually install TalkBack. I'd then look for something to link installed apps to the Play Store so you'd automatically get TB updates. I know Titanium Backup has that ability somewhere in its long overflow menu. I think it's called Market Doctor or Market Linking.

Also, make sure you don't need a newer baseband, radio, or whatever Lineage calls them (I've seen both terms.) Those are generally shipped with official software updates, but I'm not sure they're part of Lineage. I've occasionally gotten burned by upgrading my ROM but not the baseband, then things get into an unbootable state as you've described.

Wish this wasn't so fucking wild west. I want to buy a phone and install whatever OS I like on it, just as I do with my PC. While I'm at it I also want a unicorn... I suspect PostmarketOS/Pinephones aren't going to be accessible to us for a long while.

How do I wipe the system partition? Typing "twrp wipe system" doesn't work. Neither does "twrp wipe cache".

Running fastboot -w doesn't seem to help. All I get is a message about wiping user data, but this doesn't seem to erase the system partition. When I boot into TWRP, I'm told the /s partition is still full. How do I erase it?