Had 2 customers do it here in NY/NJ and someone else in FL. Not mining related, just manufacturing

Saved them a lot.
BUT:

1) Everything is 'your fault' not the fault of the power company until you can prove otherwise when it comes to low power issues. 3 states, 3 different power companies, same BS.
2) If you do have issues they will not even test TO the transformer without your electrician being there.
3) If you do need parts / repair you can be down for a while. Unless you have a spare $50k transformer sitting around like the power company does.

For my customer that had a lot of issues it was stressful and annoying, but they could function on a limited bases.
A mining facility is going to be off....

-Dave

I don't think services are taxable in most U.S. states. You're probably providing a service, not a good. If not, then try structuring your business so it's obviously a service.

Think of it like a cloud hosting provider. I've never paid sales tax on VPS hosting even from providers based in the U.S., even though both parties live in states with a sales tax.

Personally my business model will not be to sell hashrate or mining contracts. I would set up a new corporation for the customer so the full tax benefits of depreciation pass through to them. The customer would be a non-voting 100% owner of that corp. My parent company would charge the mini-corp a fixed $/kWh rent rate or something + a setup fee. Whatever profit is left is paid out to them as a distribution every month or so. Still need to talk to a lawyer to make sure this is a sound idea.

Sales tax on electricity: This is much harder and you need to be larger-scale, but some states (like Texas) have exemptions to the 6.25% sales tax for datacenters and other companies like that. Oklahoma has a similar bill coming up.

... I have the financial stuff, like how to avoid income tax & sales tax, under control thanks to my accountant.

I would think that an experienced GPU miner might be a little turned off to ASIC mining for a few reasons.  ASICs tend to devalue faster than GPUs in my opinion, so while their volatile prices might give you more opportunities for profit scenarios, you have to be quick to act because you can be left holding worthless bricks in the end if you aren't nimble.  There is also more dependency on a single currency, whereas GPU mining gives you added flexibility if one coin's price dips or something.  You're also left with less usability for your equipment, so if regulation or market conditions causes ASICs to become worthless, GPUs will still have buyers in gamers and graphic artists.  Personally, I think GPU miners will be a bit disappointed moving into ASIC mining, but if you pick the right entry and exit points, it can be massively profitable.

I wanted to go with the day-ahead index, where I always know what the rate will be in 24h and it's not subject to instant spikes like the real-time LMP. I'm not worried about the spikes because I will write custom, automated software to cut off the Internet to the ASICs to power them down. Maybe other farms will want the software.

Can you elaborate why and what is your use case?

150kw is getting into 3-phase territory but still very practical for single phase service. It comes down to cost of the panel (size and amount of copper in it) and if the power company needs to supply you with a different service transformer to feed the building. As Mikey said, go with what is cheaper.

You're over-thinking things....
Unless you are pulling mega-watts of power, load balance on the utilities transmission lines is not your concern. If you have 3-phase power you only have to reasonably balance power from your distribution panel. If you have single-phase power it is still the utilities concern - not yours. Of course this also assumes that you are in a fairly well developed country with stable & modern power grids.

In most countries 3-phase is typically delivered to a building as a wye connection with a center Neutral connection - not delta. However, that also depends on the local power loads. Wild leg or high leg grounded delta is usually only found where most area loads are single phase and only a few select customers need 3-phase. Unless it is your only choice, due to the voltage imbalance between the wild leg and Neutral I do not recommend grounded delta.

What country is this in? How power is distributed varies between countries and even between regions within a country.
What voltage is your typical wall outlet at home or business? 110v or 220v?
ref https://en.wikipedia.org/wiki/Three-phase_electric_power

If you want 208-240v 3-phase power, and if the location also needs a lower voltage for wall plugs & lighting just tell the utility that. They will decide on the type of transformer used as it all depends on how they distribute power in the area where you are.

...
If you are in North America, depending on what kind of power is needed for other businesses nearby, their '3-phase' may not exactly true 3-phase but rather 3-phase created by using a 'wild leg' as referenced to Neutral. Voltage across all 3 or any 2 lines is the same. Voltage from either L1 or L2 referenced to Neutral is balanced to 1/2 the L1-L2 voltage. For 208 VAC that is 110V for L1 or L2 to Neutral. Voltage on the 3rd or Wild leg referenced to Neutral is much because it is is NOT reference to Neutral. The 3rd leg is supplied only to places needing 3-phase to run motors, refrigeration, and other application requiring it that are powered by all three phases. If that is the case the utility is already supplying single phase power over very heavy lines feeding several other locations nearby.
...

150kw is getting into 3-phase territory but still very practical for single phase service. It comes down to cost of the panel (size and amount of copper in it) and if the power company needs to supply you with a different service transformer to feed the building. As Mikey said, go with what is cheaper.