Touchscreen Intermittently Working, Screen Issue? Board Issue?

I did a search, but didn't see this particular problem listed.

I have an iPhone Xs (Model #A1920, Software Ver 15.4.1) that I bought about a year ago, refurbished, on BackMarket. Phone immediately went into an Otterbox with a screen protector. No cracks, no chips, no visible damage, and it hasn't been dropped in water or otherwise abused (at least not by me). It's pretty much in the same condition it was received in.

About 7 months after I bought it the screen developed an issue where it would randomly go unresponsive, and then just as randomly it would start working again. Here's a video I took of the issue back in December:

https://www.youtube.com/watch?v=SIImbKog...

I sent it in for a warranty repair, it came back working, but a few days later the issue was back.

I sent it back again, and it came back working, but about two weeks later the same problem occurred.

Sent it in a third time, and again, the issue came back after I'd had the phone for a few days. However, this time I dithered about sending it in a 4th time (I'm out about $60 in shipping and screen protectors at this point), and the issue kind of went away after about a week.

And it has worked flawlessly for about three months, but as of last week, the screen has been acting up again.

For example, there are times when I can tap the screen, while the phone is resting on a surface, and it will wake up; but if I pick the phone up and try to swipe up the screen will not respond. There are other times when I tap the screen and the screen stays black, until I lift it up; at which point it will wake but the screen won't respond to touch.

There have also been times where the screen will swipe up, and it will work for a few seconds (e.g. long enough to open an app) before freezing again. There are even times when it works for 20-30 seconds before freezing; which is usually just long enough to think I can accomplish something, and start trying to accomplish it, only to be thwarted partway through.

The periods of unresponsiveness can last for minutes, hours, or most of the day (i.e. most of the day means I have checked it periodically throughout the day, and each time it has been unresponsive).

Today, the screen stopped working a few times, but started working again within a few minutes, and has, for the most part, worked all day long.

Battery charge level does not seem to matter, temperature doesn't seem to matter, I've done a hard reset, set it up as a new phone, etc. I haven't been able to pin down any circumstance in which it works more/less consistently. Just kind of works or it doesn't as near as I can tell...

Only other thing that 'could' be related? The wake/sleep button on the right side of the phone doesn't work consistently. There have been times when I've had to press it multiple times to either wake/sleep the screen.

Trying to figure out what to look into to get the phone fixed. I don't want to dump a bunch of money into it (e.g. buy a screen and find out it's something else, or pay someone and have another botched repair job, etc.) unnecessarily.

Thoughts? Thanks.