Will I regret not being able to zoom 10x :(

Nobody can tell except yourself. Rent the D5500 with the AF-S DX 18-55mm VR II and try it out.

I really like the vary angle LCD thing and the fact its lighter and has wifi built in

Just like big optical zoom, these are not exactly the main features of a DSLR.

You should consider all features of both systems and see how they impact your photography.

Here's the beginning of a comparison table, that you should finish on your own. (both Nikons with the lens mentioned)

                SX 120 IS              D3300             D5500

year released   2009                   2014              2015

weight [g]      245                    410 + 195         420 + 195

f.length [mm]   36–360                 18-55 (27-82 with x1.5 crop factor)

max aperture    f/2,8–4,3                       f/3.5-5.6

                                      .
                                      .
                                      .

Now let's put some of this into perspective:

the fact its lighter

The difference in weight between the D3300 and the D5500 is 10g. This is nothing compared to the fact that they are both over twice as heavy as the SX120. If you pick up all 3 cameras, you won't notice the difference between the Nikons.

the vary angle LCD thing [,...] intend to use the camera for [...] nature

Chances are that the sun is shining in nature, which renders the display useless. It can help getting different shots from unusual angles and/or macro.

If the sun is not shining, it's possibly raining. How's the weather sealing of those cameras?

astro shots at night

For that, you need a wide lens, open aperture and high ISO. It's one of the most demanding applications for photography. Zoom ain't worth anything here.

Depending on how serious you want to get, interchangeability of lenses gives you the possibility to mount the camera to a telescope.

For both astro and nature photography, a gps appears to be a possibly desirable feature.

etc.