The subsidy isn't the problem per se, it's the net increase in income.
It is obviously self-evident everywhere that high incomes create high cost of living, which can be traced through higher costs all the way down to the land rents (the rent someone is willing to pay to have market access to the high local incomes).
I don't want a higher income, I want to benefit from the productivity gains I and everyone else made happen by having more time to do things I like.
Why don't you just do that now and work half the amount of hours you're currently working?
Show me the job like mine where this is an option, and I'll take it in a second. Hire another me and we'll split duties.
These sorts of "professional job that pays a professional hourly rate but is for 20 hours a week" are exceedingly rare. You'll usually be taking far less than 50% pay - far worse if you include benefits in the calculation.
I've been halfway keeping my eye open for such an opportunity so I could fund the basics of my life, plus have time to do personal projects with utterly no chance of monetary payback. Just stuff like paint the house, teach myself how to weld, work on backyard art, volunteer, etc.
I could certainly find a job that pays 50% of what I get now for working the same number of hours though. Perhaps moderately less stress and no "off hours" chance of being called in for an emergency. But that's not a great tradeoff since I'm looking to trade money for time.
This may not be the point you're making, but it really is sort of frustrating this isn't an option. I get why - I employ folks too and understand the overheads involved - but man it's the dream!
And that's my point.
Your purchasing power will not change.
Your purchasing power is defined in a competitive equilibrium with your peers.
If you're assuming you can band everyone together to all decide to work fewer hours for the same pay, fine, but you just invented a union, not an improvement to UBI.
A Universal Basic Income gives everyone a flat monthly or bi-weekly income. Whatever jobs you work on top of that also provide you income.
As a standard W-2 employee (in US terms) this UBI payment would be factored into your W-4 paperwork (income tax withholding). As your wage increases your withholding increases as well and at the end of the year ideally your return has a clean net 0 under/overpaid.
Below some income threshold your total income tax contribution would be less than the UBI payments and so you'd be receiving a prorated negative income tax throughout the year. You could also call it a prorated tax credit or fixed disbursement social welfare grant or whatever.
At that income threshold you are receiving an interest free loan from the government for the year with loan disbursment on a fixed schedule throughout the year. And of course you promise to pay back in full by the tax deadline (either via withholding and/or with a lump sum at the end of the tax year).
Above that income threshold you are still receiving that fixed disbursement schedule interest free loan from the government but you also start paying additional income taxes on top of that loan. This is of course all still handled via W-4 deductions during payroll and nobody touches your regular UBI disbursement that shows up in the bank as a direct deposit or as a check in the mail. It still shows up every 2 weeks or every month.
But importantly this system is resilient to sudden changes in income. If your income suddenly increases, you factor that in via your W-4 and nothing changes. But if you suddenly lose your job or you move to a much lower paying job, you keep receiving your UBI disbursements on that fixed interval and you aren't left with a tax burden for it at the end of the year.
And so UBI as a system is purely an implementation detail. If we took existing welfare systems. Housing subsidies, food security subsidies (SNAP, etc), insurance subsidies, etc. We factor their per person cost/payout and roll it all together into one fixed interval UBI check. We keep income tax rates the exact same as they are now but shifted to factor in this UBI income (i.e. start everyone at a negative income floor that slowly gets filled to 0 dollars once every UBI check for the tax year pays out). The taxes paid and the net incomes for everyone stays identical (more or less due to variations in thresholds for existing benefits programs).
So at the end of the day your income stays the exact same but there's more money moving around and more consistency for the tax payer/citizen/resident even when suddenly life events change their financial situation.
/rant. sorry for the wall of text