Hello - welcome to the forum!
A remap / bluefin or digital tuning box - probably makes the biggest difference "bang for buck"
Fitting an EGR blanking plate can be benificial, there is a lot of info on this on the site
There is quite a lot you can do, either cosmetic or power enhancing etc, if your car has a DPF you can even get it phisically removed, electronically deleted, a stainless steel exhaust fitted and a "custom" remap done at once from the same place, members have done all sorts of modifications to their cars, ofton on their "signatures", worth having a look round
After using a tuning box for years (high end digital DTUK) I decided to switch to a Bluefin in the search for more power (and out of curiosity - to see if it made any difference)
The tuning box was adjustable (an earlier single-map one, some of the later ones had multiple maps) and had a custom remote adjustment fitted - the car was tested for 0-60/ midrange etc and the box adjusted for maximum acceleration (along with the adjustable digital boost controller)
The car has many other (performance) mods - too many to list
When the bluefin was 1st fitted, the top end power/ revs were better, but the bottom end was not as good/ strong, Superchips were very helpful, and after an interim map, the 3rd map has almost the same bottom end power as the tuning box but better top end power / revs
Im hoing to get a dyno / rolling road done soon, i have a limited amount of runs but may compare the different setups (Bluefin/ tuning box etc)
Superchips were doing a 20% discount so the £399 turbo Blufins were about £320, i know you can get remaps frome elsewhere for less than that but i recommend superchips/ Blufin,
Typical power improvements for a stock turbo-diesel are around 30Hp and the MPG is often improved too, the handset also works as a code reader, (and immobiliser!) and the map can be swapped back for the stock one (for the snow, etc )
Suposedly a "custom" remap with before/ after dyno runs should be better than a Bluefin (that is supposed to be "generic", and not suitable for modified cars, or heavily modified cars) , but im not convinced, and there may not be much (if anythig) "in it"