ejector pump siphoning traps on 2nd floor

As per previous threads, I have had a problem with sewage smells coming from the ejector pump especially after it is left for a week. It is more noticeable i winter due to negative pressure but sometimes in summer.
I spent a lot of time trying to seal cracks in the old lid, which is 21" (only 18" sold nowadays). I then did a smoke test and found smoke coming from between the liner and cement, which must mean there is a leak under the ground or where the main waste pipe goes into the liner.
So, there are 2 problems:
- smell / air leak
- siphoning traps

I plan to completely replace the liner and lid and re-cement as the liner is 25 years old. However, I'm still worried that there will be a siphon problem when I do this. How can I solve that? The house has an upstairs toilet plumbed directly into the septic tank and it seems this is the one that gets siphoned (maybe it shares the exit pipe with the pump discharge pipe)?
It is worth noting that there is no main vent stack in this house. I cannot tell if the toilet and other pipes have AAVs but the only vent going up to the roof is a 2" vent that comes from the main ejector itself.

Also,
1/ if I replace it, should the basin be 22" or 30" deep for sewage?
2/ How thick is the concrete layer likely to be to break it up? and
3/ where is the waste pipe likely to be? set into the concrete layer? Might I risk damaging it when I break up all the concrete?