They are very simple to install(but do get someone who specialises in the installation)l, can go almost anywhere even up hill(with pump), under a drive.
You don't need to have them serviced , but it is wise, and easy, so pay attention when it does get serviced, as you can easily do it yourself once you have seen how to clean the filter).
They work perfectly and stress free and never smell, if you are strict about what you allow down them, You are trying to keep perfect bacteria just like your own gut, so no tomato skins(if your body doesn't digest something neither does your tank( septic tank/or klargester) , bleach, disinfectant, just cover type products, if you stick to that it works perfectly and you forget about it.
Where we live the environment agency have ruled we have to get rid of septic tank and update.
I guess if I did a geographic survey of your land you would have soft ground over hard (simplistic explanation) which with all the heavy rain is holding a lot of ground water, which means the septic tank has no run away.
I assume you have looked in all your manhole covers?
You could also get a pipe survey, from all your stack pipes to the septic tank, we always use good local company, who will stick a camera up and down all pipes, and check , could be crack related, could be bad repair (think concrete tipped over crack) could be someone has flushed something bad.
Process of elimination , and check your insurance cover, some do include drainage/tanks.
But up grade
If you do stick with septic tank, and the ground water goes, it won't hurt to shove a dead animal (usually dead lamb) in to get the bacteria going.