There are now cheap eSIM deals for international travel, but old phones and even budget new models don't support eSIM, so it appears you are stuck buying travel SIMs, but there is an option.

eSIM modules

They are like a SIM card, but are reprogrammable. You need to use an app to write to them, and not all of them are iPhone compatible, but once the SIM is programmed you don't need to do it again. They normally allow you to store many sim cards, up to 15, but you can only use one at a time. I suppose this could be handy if you only have a single SIM phone, then you could swap SIM without needing to eject the card.

I have an old Xiaomi 11t, it has dual SIM. One is my home card, the other is a Switch SIM I paid £15 for, other cards are available, but this was the cheapest at the time I was looking. There are cheaper options such as this but i haven't tried it.

iPhones have had eSim built in since the 11, you would hope so at the price point, so there's not much point in spending extra for the iPhone compatible modules, unless you want double eSim I suppose.

Roamless

Roamless offer an eSim service, there are others similarly priced, but this is the one I settled on using. It costs around $2/gb for the places I find myself. I have used it in Japan, USA, China, Hong Kong, and Indonesia without issue. It also allowed me access to Gmail in China without a VPN.

Ad Blocking

The eSim data is cheap, but it's not as cheap as my standard Sim, so it's worth reminding that you can block out the ads with nextDNS private DNS to save on some data, maybe use PipePipe too for YouTube so you can download videos over Wi-Fi.