Please make a decision regarding the hot water supply

Hello everyone,

Last year we bought a two-family house, in which two tenants had previously lived. One of them was ourselves (for 8 years), living in the ground floor apartment. After the purchase, we still had a tenant on the first floor until May this year. The tenant has now moved out and we are currently converting the house into a single-family home (thus merging the apartments). The first floor has already been renovated (completely refurbished). The ground floor is to follow next year, as soon as the first floor is finished and thus habitable (then we can move upstairs and completely mess up downstairs ;-).

What is still missing on the first floor is the hot water supply for the bathroom. The kitchen can be ignored for now, as it no longer exists and is being converted into a dressing room. In this context, we had the existing cold water pipe professionally decommissioned.

Regarding the hot water supply, we had the following situation so far:
First floor: Instantaneous water heater in the bathroom (shower only) and boiler in the kitchen (under-counter unit)
Ground floor: Hot water branch line from the basement (no circulation) for bathroom (bathtub + shower) and kitchen
Hot water generation via oil burner from 2004, regularly maintained (Weishaupt, currently satisfied with it). Hot water storage tanks were renewed this month. It currently takes between 10 and 20 seconds until hot water is available.

The new bathroom upstairs on the first floor has two outlets: bathtub + washbasin. The future second bathroom on the ground floor will also have two outlets; shower + washbasin and will also serve as the guest toilet ;-).

What should I do now regarding the hot water supply on the first floor? I have already considered the following options:
- Electronic instantaneous water heater (e.g., a Stiebel Eltron unit)
- Branch line (similar to the bathroom on the ground floor) laid up
- Circulation line installed.

Branch line:
Pro: Easy to install, low space requirements, low cost for installation
Con: It takes a long time for hot water to be available (currently between 10 and 20 seconds on the ground floor, but this does not bother us), possibly not legally permitted due to length of approx. 10-12 meters

Circulation line:
Pro: Immediate hot water
Con: High initial effort, high initial cost?, large space requirement, possibly cost issues during operation with oil burner if oil prices rise again?

Instantaneous water heater:
Pro: No pipe laying from the basement
Con: Electricity costs, cost of a proper instantaneous water heater

Of course, it is possible that the apartment on the first floor will be rented again in 20 years, but this criterion should initially play no role in the decision regarding billing capability & co. Both bathrooms are located one above the other. The kitchen on the ground floor is directly above the heating system.

What is your opinion or your recommendation on this topic? I would be grateful for any help or tips!

In another thread something similar was discussed (search function?) For example, there is also the alternative of the branch line with an instantaneous water heater, so that warm water is immediately available, but if you need more, it is not heated with electricity again. The operating costs are quite low because a) if you only need warm water for a short time, you don't waste anything (in a long line) b) if you need it for a long time, the instantaneous heater switches off again.

In our old building, we had similar problems. In the end, we installed a tankless water heater in each bathroom and an undersink unit in the kitchen. The new owners stuck with it despite some renovations. The electricity costs were not very high, but hot water was quickly available. There is a manufacturer in Lüneburg who is very good. ;)