The UK SIM is a single UK SIM from 3. I bought it from a vending machine at LHR Terminal 3 in January 2020. Coincidentally, I've been using LHR as as a European entry point since then, so I've been able to top up. If I lose that number, it's not a big deal.

I am on the original Simple Choice plan due to a promotional add on that I would lose by changing plans, so I don't get high speed T-Mobile data when roaming internationally (even in Canada and Mexico) except the 5 GB for T-Mobile locations. I have international stateside calling for $10/month across all lines on my account. It's normally $15/line per month.

As for Australia and Taiwan, Mrs. Majuki's sister is in Sydney, and we typically make the trip there once per year. It's handy for friends and family to have the same number. With timed reloads, I only burn A$10/year, if that. (Some years I have made two trips.)

The in-laws, my engineering colleagues, and some customers are Taiwan based. They all have my Taiwan Mobile number. To maintain that number and the prepaid balance, there just has to be activity once every 6 months. Like with Australia, I can time activation of a data package with our annual end of year trip. During the summer, I activate a 1-hour data package for NT$30. Even without a trip there, such as for 3 years during the pandemic, US$2/year was worth it to maintain the service. Coincidentally, both Mrs. Majuki and I switched our Taiwan Mobile service to eSIMs in January this year.

Our trips tend to be longer in duration, so having local SIMs makes sense. Furthermore, T-Mobile frowns on roaming for extended periods of time. Our post-pandemic trip to Sydney last year spanned 3 billing cycles. Roaming for that trip would have earned each of us a strike having roamed 100% for one billing cycle.