How to fix erratic touchscreen on iPhone 7 with iOS 15.8.5?

My iphone7 (software version 15.8.5) recently had problems with the touchscreen including in whatsapp. I would type one letter and one or a whole string of letters or numbers appears, then delete doesn't work.

The screen looks clean and I have cleaned it again to be sure.

I found several videos showing that you would press one letter such as "u" and a "j" appears. My issue is more serious than that more commonly known issue at worst I spent 15-20 minutes trying to do "Good night" and then gave up and went to bed with the screen showing 50 random letters I couldn't delete.

Eventually the phone would prompt to put the 6-digit code in but that would also register wrong letters so it would lock me out. This happened about three times around 11pm so I just went to sleep and the next day it worked normally.

The issue happened on holiday a lot. The issue happened more when the phone actually felt warm to the touch.

Turning the phone off or not using it for a few hours helped. Turning it off for a few minutes didn't work and sometimes I wasn't even able to turn it off because the touch wasn't working.

One of the reasons the phone was getting warm was because I used it for Sat-Nav for several hour stretches on holiday when it was exposed to the sun.

A part of me thinks that the phone felt warm because I was using it so much and it was exposed to the sun, and that was quite normal. But on the other hand it was plausibly getting warm faster than usual, I can't quite decide. Now back home after the trip with lower usage and it's cool, normal.

I did wonder if the issue was cooling system failing like the fan failed in my laptop once and required cleaning however googled it and looks like it has no cooling system as such in iphone 7 or certainly no fan so perhaps that can't be it. Or maybe it's nothing to do with heat.

Since I've got back from holiday I've been mostly indoors and the issue hasn't recurred as often but hasn't completely gone away either.

Obviously an iPhone 7 is a very old phone (although it was bought brand new 3-4 years after it was first released so isn't as old as you might think perhaps) and it's done well to get this far, so I am pleased with the phone but I've already thought of buying a new phone so no need to suggest that. Before I take that step, I am more interested in whether there is any other solution. However, aside from that, I would really appreciate any other ideas, thank you for reading as well.

Maybe if I get in the habit of having the phone off for a hour in the day time that might help?

Perhaps go to a shop and get them to clean the insides and tinker around, although seems like it could be more of a software issue.

Anything else?

[Re-Titled by Moderator]

Original Title: iPhone 7 touchscreen erratic