Next time can ping me like this
TOS
You type '@TOS' with square brackets around it.
Yea, the cheapest way to park your funds in IBKR is just use BIL ETF. Park all your USD save for a couple of dollars for commissions in BIL ETF. Withdraw anytime during US premarket/aftermarket or during normal trading session then you can use it to purchase any US stocks/ETFs instantaneously, even with cash account, no need for T+1 settlement unless you need to withdraw the money out of IBKR account into say, your USD bank accounts.
Try to avoid fractional shares with BIL ETF yea... At worst, you will leave behind some 90ish USD as cash in your IBKR account. Fractional shares for stocks with 4 digits share prices are easier to trade, but for double digit unit price like the case of BIL ETF it's a hassle to buy and sell. After all, your goal is short-term liquidity management here...
As for tax, indeed as
Medufsaid said, there is a 30% WHT when BIL credit the dividends to you every month, but around early February of the following year, IBKR will reverse the WHT and refund you the money in your USD cash account.
The expected return of BIL ETF is about 20 basis points below the 1-2 month US T-bill yields, mainly due to the 0.14% management fee charges from SSGA and another 0.05% from the lost in interests on interests due to the 30% withholding tax.
*In case you wonder how to compute the lost in interests on interests... here's the maths:
Assuming you start in February in year 0, you lose 5%/12 * 30% WHT * 5% , the second 5% being the interests on interests. In March, you lose 5%/12 * 30% WHT * 5% * (11/12), now you lose (11/12) of the annual interests on interests (you only got 11/12 of a year left to compound your interests on interests until next year's February, when IBKR refund you the 30% WHT), but the capital remains the same, i.e., 5%/12 month * 30% WHT. So the total opportunity cost, in simple interest terms, to first order effect (we ignore interests on interests on interests and .... on interests... and...

), the sum of series is 5%/12 * 30% WHT * (1/12) * (12+11+10+...+1) = 0.04% p.a.
Do correct me if my maths is wrong... my head is spinning all over the place writing my PhD qualifying exam report...

If I'm buying, I think most likely in batches of 100s, but looking at the bolded part, the interest is given in the form of shares, thus the fractional shares? Is this BIL similar as SGOV?