1. The tumour is locally advanced, unresectable pancreatic head adenocarcinoma (M0) because it involves 200° of the SMA (>180°). The 90° SMV contact alone would not make it unresectable. Upfront Whipple Procedure is not recommended because an R0 resection is unlikely and morbidity is high. Neoadjuvant systemic therapy with restaging is preferred.
2. He presented with bulky, pale, floating stools and weight loss which suggest exocrine pancreatic insufficiency. I ideally would start pancreatic enzyme replacement (pancrelipase), about 40,000–50,000 units of lipase with each main meal and a lower dose with snacks, taken during meals. Titrate according to stool symptoms and weight. Counsel him not to unnecessarily restrict dietary fat and to take enzymes consistently. Because he is self-funding, I will use the most affordable reliable preparation available.
3. If he is ECOG 0–1 and medically/nutritionally fit, I would favour modified FOLFIRINOX because of its greater systemic efficacy in fit patients with locally advanced disease.
Gemcitabine/nab-paclitaxel is reasonable if he has poor nutritional/functional status, significant comorbidities or neuropathy, or if FOLFIRINOX is impractical because of cost, infusion-pump requirements, admission requirement in my setting or travel.
His bilirubin has improved substantially after drainage, but liver function and drainage should be reassessed before chemotherapy.
4. I would reassess the cause and severity of pain and introduce opioid analgesia with laxatives for persistent moderate-to-severe cancer pain, with regular review and management of constipation and nausea.
If pain remains poorly controlled despite appropriate analgesia, celiac plexus neurolysis, can provide additional pain relief and may reduce opioid requirements.
In summary, no upfront surgery, optimize nutrition/PERT and biliary drainage, systemic therapy (preferably mFOLFIRINOX if fit) and aggressive pain control. Afterwards restaging and reassessment for possible conversion surgery.