first, you perhaps meant to say bill went from 10k to 3.3k
secondly, I'm positively suprised that the solar energy you produce at your house it's for your use free, only the excess you give to the network has a price... at least that's how I read your post. In social EU they invented a more "social" sharing where whatever energy you produce is accounted as sold to the network at fixed gov published price, but what you use, even though it might be entirely your energy is accounted as buy from the entire network
Why would it go from 10k to 3.3k? it's normal for electricity bill to go up significantly when switching from oil to electric heating. Then you use solar panels to bring that cost back down.
I've heard about some of this stuff happening in the EU, but I'm not sure how anyone can keep track of that. The smart meter from the city can only track incoming and outgoing electricity. When I'm using my own solar power, I don't import that power from the city. To them, it just looks like I use less power. If I'm at net 0, they have no way of knowing if I'm not using any electricity at all, or if I'm producing 8kW and using it all myself.
A quick google found the following:
The recommendations have gone to deeper bores - it may depend on the local geology. Shallower is better for cooling in summer.
The bore should last up to fifty years.
I reached this conclusion as in summer, I don't see the furnace turning on when I have a hot shower...
Re replacement - agreed, but I don't think it makes sense to replace a working oil heater for now... I was wondering that, because I've been told that I should do it "now, before the government incentives end".
To be honest, what seems more interesting for me are solar panels. Similar cost to a heatpump, but from the rough indications I can see, would drastically reduce my electrical costs... From what I can see at 40k installation costs, and an electrical reduction of ~1500/year, that would be 27 years to repay for themselves.
Considering electrical prices are bound to go up with the lack of proper generation facilities; and solar panels seem to have a life expectancy of 25s, it could almost make sense...
Maybe if the cybertruck arrives
You may be underestimating the savings from solar panels and overestimating the installation cost 
We had offers that varied significantly, from 25k to 40k. Seems all the big companies took the inflation opportunity to adjust their prices, while the smaller companies are still trying to stay competitive. In the end we chose the 25k one and ordered more panels than initially planned, bringing the cost to 30k after tax. There is a ~5k rebate, and the entire cost can be deducted from income which is another ~20% of that, but this varies on the locality. All in, we’re looking at around 20k for a 13 kWp installation (32 panels spread on roofs that get light at different times of the day, so we’ll never hit that peak - it maxes out at around 9kW).
In terms of saving, the biggest savings come from using your own electricity. This means that the biggest power hogs, like hot water, laundry, dish washer, should be done when it’s sunny. It also means that you need to have a way to turn on the boiler when there is excess solar production. Had I not switched to electric heating a few months after installing solar panels, my electricity bill would have gone from around 1500/yr to negative 500-1000. If you’re lucky enough to have a roof which is ideal for solar panels, the saving potential is very surprising. My 20k investment will most likely be paid off in 10-15 years and will continue to pay for the 15 years after that.
Here’s an interesting resource which gives you an idea of how much actual production you can expect in your area, based on kWp. These are sites where the owners enabled public access, sometimes the name even has an exact address so you can look it up on google maps satellite to see how it looks.
https://monitoringpublic.solaredge.c…c?locale=en_US
If you have a car that sits in the garage all day, not used to commute to work, it could be interesting to get something which supports V2G (vehicle to grid), where it will be used as a battery to store excess solar production. Then you don’t even need to do things when it’s sunny. Obviously not a good investment to buy a car for that, but if a car needs changing, could be something to consider.
Maybe I’m a bit of a nerd but I find this solar stuff very exciting
Heat pump PLUS solar is the obvious option - that's my plan anyway; heat pump first, solar later, in different years for tax reasons (and I can't afford to do both at the same time anyway).
Cooling via reverse use of normal heating units (i.e. underfloor/radiators) doesn't require any additional permission, at least in SG.
In the detail, there is a ton of "fluff". it had 590chf just to get priority on their inventory , and 840chf for network communication connection to their cloud monitoring. Sorry, I'm not paying nearly 1000chf just for them to connect a Cat.6 cable to my router.