So it's just a simulation isn't it?
All your secondary circuit, bridge, filter, load is floating, i.e. it has no reference to 0V node.
Most simulators are not happy with this.
So I didn't read problem's data carefully enough and I apologize for this.
The matter is much simpler: it cannot work.
Switches: a 500W system run from 12V would take over 40A. Just rds(on) and primary windings resistance score up to 18mohm: this alone drops nearly 1V from 12V and dissipates over 30W statically only. Then add switching losses
Moving up to 24V or 48V would be a very good choice unless really impossible.
In any case such an inverter will take several MOS for each switch and for sure transformer primary winding will not be made of 1mm copper wire. Wound copper stabs or several parallel windings are mandatory.
Talking of transformer: so far it is no clear to me wether we are talking of measured or simulated data but inductances tell that coupling between primary and secondary is very loose.
With Lpri=3mH, Lsec=1.9uH and perfect k=1 coupling Lm should be around 75uH. If you have 27uH it means k=0.35 or so and hence around 2.6mH of series inductance.
Around 1kohm reactance at 60kHz.
First you need a much better transformer, but IMHO inverter design is also a far cry from a good working circuit.
I swapped Lpri and Lsec but it's just the same, they are very loosely coupled, they cannot transfer much energy each other.
How have you designed it? What is the core size and shape? Where have you got those inductances?
Simulators are toghether the most usefull and the most dangerous to any field of engineering.
I am afraid that used with no background experience, sensibilty to results and experimental work will not give any good results. By the way designing SMPSU transformer is one of the most demanding tasks. It is made of educated guesses, trials and errors back and forth electric, magnetic and thermal design at the same time.