1+2: this is possible - I even believe that the Ethernet port does not put more load on the CPU than the SFP+ ports - if you look at the block image at Mikrotik ( https://cdn.mikrotik.com/web-assets... ), Ether1 is also connected to the switch chip.
3: I don't know of any SFP transceiver that has a quasi extension (i.e. SFP - cable - Ethernet). But there are 2 possibilities: Either you use 2.5G Ethernet SFP+ transceivers, they are much cooler (but you don't have a 10G network ;-) - or you make sure you use 10G SFP+ transceivers that run extra cool - there are a few that only need 1.8 W (with Broadcom chip) - often they are the ones that have 80 m or 100 m range. The 30m versions are usually the 2-3W ones and they get incredibly hot! Unfortunately the low power ones are more expensive: There are some at fs.com or apparently a Wiitek (I haven't found one in CH unfortunately) - there is a thread where a few are tested: https://forums.servethehome.com/index.php - I bought 2 from AliExpress but haven't received/tested them yet (ZYOPM and NVOC).

The Ethernet port can be used, however it’s not recommended as it is wired into the CPU and increases the load on the processor.

There are DAC cables for short runs which plug into SFP+ ports at each end, this works with other routers/switches as well as computers equipped with a SFP/SFP+ port.

Putting RJ45 GBICs is possible, but in 10G they do run hot so you must avoid having them side by side, and you should factor in the increased power usage and add a beefier power source.

There is a CRS304-4XG-IN fanless router/switch with four 10G RJ45 ports, unfortunately no SFP+ cage.

Many thanks to both - kussmaul & ezdiver.

My conclusion from this:
a) Do not use RJ-45 with this product,
b) Avoid RJ-45 with transceivers if possible, especially at 10 GBit/s.
c) SFP+ DAC conceivable where necessary (e.g. PC later).
d) Broadcom chips are cooler & possibly more expensive. (Link)
e) Power consumption of the transceivers is different, as is their heat development.
f) Transceivers can be expensive (please be careful).
g) This product (or smaller ones like CRS304-4XG) would then tend to run at the limit - so, unsuitable.
h) Min. 1x RJ-45 1Gbit/s remains indispensable.

My goals (not urgent) for the house are
- to ensure fibre optics vertically on three floors with SFP+ possibilities.
- The old Rj-45 cable is to be replaced in the wall with several fibres.
- Find suitable devices that operate at least 4x SFP+ and 1x RJ-45 permanently.
- Fibre modem/router and central switch should be possible on each floor (therefore 1x SFP+ more than floors).


I don't know enough about this, but I think that a high switching capacity indicates that the device can cope with a lot.
That's why it makes more sense to use half capacity than full capacity. I would consider the following new products (only sorted by price, open to others):
1) Item: 44973940, Grandstream GWN7832 (manufacturer unknown to me)
2) Item: 18712968, Zyxel XS1930-12F (recently bought a smaller product (again from) Zyxel, known)

XS1930-12F is much more expensive, but "cleanly constructed" as a solution; SFP+ only intended for 10 GBit/s with sufficient distance between each other and 2x RJ-45 for e.g. 1x fibre modem/router directly and 1x rest.
GWN7832 is cheaper with many SFP+. If necessary, I will ask about the product there.
I have nothing against the CRS309-1G-8S+INL here, it just doesn't seem to be suitable enough (for me/our house) for a corresponding central location.

Thanks again to both of you, I'll keep that in mind.
Zyxel would simplify a lot, just the price is a lot...