Venting issue?

Hello ,
Looking for some diy ideas before I call in the pro's.

I bought a small cabin a year ago and have had this same issue since day 1.

Flushing the toilet: started out as in intermittent problem. I finally narrowed down the intermittent problem to the following.

Every time the shower is used (no bathtub) it would require the bathroom sink to be plunged, and presto.. The toilet would flush with great pressure. This process would be required every time a shower is taken.

I have a raised septic that is fairly new (2years old) and there is no baking up issues anywhere else. Gurgling does occur when flushing toilet in both shower and bathroom sink.

I have tried the following. Plunged the toilet, shower, sink, pulled the sink trap, and looked for roof vent clogged.

I have also researched this unusual situation and can not find any similar situations that require a "sink to be plunged after showers are taken-- for the toilet to flush properly".
The sink does not have any "add on" vents either. Cabin is on cement slab with little to no visible access to the drains in question.

Any suggestions?

Sounds like you have a clog in the piping. You might be getting some relief to the toilet branch when plunging the sink if the sink is s-trapped or has an automatic plumbing vent attached to it. You should be able to snake the drain by removing the toilet for access. Use a sewer snake, not a small hand cable.