That is certainly a better value option if you can still get the drivers and amps for a reasonable price over there.
I went the DIY route as well and build 2 of the VBSS style subs. I wanted to build something bigger, but my wife vetoed that. AVS forum has plenty of information on design options you have.
It absolutely makes a massive difference. But you unfortunately need to spend $500+ on a subwoofer to get something that outputs the full range of what you can hear. There simply are zero subwoofers below that price point with adequate output in the 20-35 Hz range.
With regards to 4k, I can understand not caring for it. I agree that for most viewing distances and TV sizes, there’s not a massive difference. However, 1080p TVs also don’t have good HDR or the wide color gamut.
Upgrading to a 4k TV with a good peak brightness (at least ~1000 nits) will be very noticable. I especially notice it in anything with fire. It looks so much better on a 4k HDR TV than on a 1080p SDR TV.
My opinion is that the next biggest upgrade is a receiver and a 5.1 (or at least 2.1) sound setup.
Did you read the article?
It’s not hours to a few days.
Take a look at how the median income in America compares to your country.