1. I see.

All solutions are difficult for me.

2. I am not sure if there is a "pay 50 USD a month; we will do all this for you" type deal.

1.  I have a trusted friend who gets what little mail I still have in the US.  If it's important, digital photos of it are forwarded to me.

2.  Search for mail forwarding services.  Just be aware that international forwarding will be a lot more expensive than domestic forwarding.  You can save money by having them send it all once per month, but that could delay time sensitive items.

3.  A certain very large email provider that's part of an EXTREMELY large search engine also has a USA phone number service tied to its email (inbound and outbound text and phone calls).  The fun part is that you need access to a text inside the US to get connected, so may need to get into live chat with a friend there to assist.  I have a US cell phone, so added my number when I was visiting the US.

If you were waiting to by US treasury bonds, I just saw a chart showing the 10 year ones have their effective interest rates dropping fast, most likely in anticipation of rate cuts and better investment alternatives to move money into.  Looks like the "buy higher interest US treasury bonds" boat may have already sailed, unless the Fed suddently changes its mind and decides to bump up rates again.

One thing to remeber - it's easier to find people to poll about salaries in Tier one cities.  Also, those who chose to answer polls about salaries tend to be reasonably happy about how much they make.  This means the numbers, while not fictional, tend to be skewed towards the high end of the range.

But, as you mentioned, lower costs of living can let someone on a lower salary save as much (or more) than someone with a much higher salary and high living expenses.  My initial salary in China was nothing to brag about, but I was easily saving a very large percentage of it.  Total salary is not the best measure of how well off someone is.  Total disposable income is where you get money for your savings.

Of course, paying down any debts, starting with the highest interest rate debts, is a fast way to free up more money.  There's little point in buying $5000 worth of bonds that pay 5% if you owe $5000 or more on a debt with an interest rate above 5%.

Even if you are paying down debts, don't forget to start at least a small emergency fund.  Anyone living paycheck-to-paycheck with no spare money at the end of the is in a hard place if they lose their job or hit some unexpected expense.

If you want some US Treasury bonds, right now (December 2023 for anyone who reads this later) is probably a good time to buy (unless you have high interest rate debts to pay down first).  Unless something weird happens, the US Federal Reserve Board has been strongly hinting that interest rates are likely to be cut during the coming year.  That means new bonds will have a lower rate and older ones (with the higher rates) will sell at a premium.

Depending on how long you plan to stay in China, the Chinese stockmarket offers some interesting possibilities.  It's taken a beating from Covid, from rocky relations with the US, and from the current overall global decline in imports and exports.  On the other hand, this leaves a number of stocks of companies that are doing an excellent job weathering these storms sitting at pretty nice prices.  If you don't want to go for individual stocks, there are all sorts of index funds available.  Don't buy stocks or funds based on potential profit tomorrow or next month.  Buy stocks or funds that have potential for profit over a span of years.  2024 is a wild card for China.  A lot of indicators show strong grown potential.  If consumer spending and global product demand go up as expected, it could be a stellar year.  If both are relatively flat, then some investors will move money to more profitable (in the short term) investments and the market may end up offering some better deals.  If the company (or the sector for a fund) is still making money steadily, I think of it as stocks being on sale.

Yes, there are many very risky elements in stocks.  Cutting edge firms offer chances at crazy profits, but cutting edge firms also have a much high failure rate than less exciting companies.  You need to have a significant amount of your portfolio in stable companies (or funds invested in stable companies), but if you want to take the risk, set aside a small percent (that you can afford to lose) for some higher risk companies.  Always carefully consider the risk before thinking too much about the reward potential.  For every share of Tesla today, there were shares of a number of other startups that ended up losing most or all of their value.

Training centers usually had children as their students.  Teaching core subjects (including English) outside of schools no longer being an option, you'll have to wait and see if one or more of the surviving companies manages to reorganize primarily around teaching adults.

This does mean fewer English teaching jobs, but the tighter enforcement of visa and work permit rules means that roving backpackers who work for super low rates are no longer a significant issue (and if you find a place that does illegally employ foreigners, there are whistleblower rewards).

So, fewer jobs - not so good.
A lot less competion - very good for those who do possess the correct qualifications.

Tell your friends that if a job involved off-site instruction (like classes for employees at a factory), they need to make certain that their work permit and labor countract clearly lay this out.  Similarly for a training company with classrooms in 2 locations - if your contract and work permit specify ONE location to work, going somewhere else can lead to serious issues.  Don't let an employer convince you that "it will all be ok."  It will be ok right up to the moment it's not ok, and that's when fines and worse start to happen to the happless employee.  It really comes down to making sure the company goes to the effort to clearly lay out exactly what the job is and how it is performed both in the labor contract and the work permit papers.  Don't let a lazy HR person get you deported.