It seems to me that your IFR, load capacity and speed requirement limit you greatly. If you combine this with the word "modern" (a relative term in aviation as in everything) then even more so. Do you mean modern design or just modern edition of an old design? As regards the former you are probably limited to either Cirrus or Diamond and if your objective is cost cutting I would have thought a Diamond Jet A1 burner is all that is left (the problems with the Diamond diesels have been done to death on here). The latter might encompass a new or almost new C172/182 - there are a few diesel powered 172s for sale at the moment, I believe.
If I had the money and an IR (neither of which I do) then I might look at the Robin DR400 with the 2.0 diesel engine. Reviewed a few months ago in Flyer. There are questions over the long term availability of spares, which links in to the financial stability of the new company which is manufacturing the spares. There is one in the UK and a chat with the owner might help.
I don't think the "sports planes" will be what you want as almost all are 2 seater and limited on load, though the Vans RV10A four seater is for sale at around £110k at the moment (VFR only of course currently). Mogas would appear not to be suitable because of your IFR altitude requirements.
I think you are very limited and you need to consider the overall cost of down sizing, i.e. what is your current mount worth and what extra will you have to spend to buy something new(er) and cheaper to run. How much flying would that cost difference give you. The saving with a more modern aircraft may be illusory if you tie up a huge amount of your capital in a new, depreciating asset.
You have not told us how many hours a year you do nor where you want to go. That would help people in giving useful comment. I assume you are a long distance tourer - in Europe?