I am no expert in the remedy of faults in diesels especially the IS220 ( and it does have more than it’s fair share of faults!)
I would say you have been lucky upto now ( prob due to your above average extended use) with the amount of problems you have had but now it has caught up with you.
Yes, everything is fixable, but the first problem is Knowing What to fix when presented with irregular running symptoms, and then it’s the Cost/Viability to fix.
Your comment about having to ‘leave the Lexus brand’ is a shame because you have had prob the worst car to ever wear a “L”badge and had a fair few good years from it, have/would you consider a petrol? As the IS250 is without doubt the best car I have had since passing my test in 1981.
Note: I also am retired and would not run a DERV car if you gave me one! As I only do about 2,000 miles a year now.