Cat sick after pooping

[heraarif]

Hello everyone. After many vet visits and a continues deterioration of my cat, I am in desperate need of advice. My vet is unable to identify what is wrong with my cat. Basically, it all started when my cat became constipated and started losing weight. He pooped every 2-3 days and strained a lot however the poop wasnt extremely hard. He is 8 years old. We took him to the vet and performed basic blood tests along with X Rays and found nothing that the vet was majorly concerned about (details of the blood report is below). Vet prescribed us with lactulose however this made no difference to the poop. So we upped his lactulose dosage and changed his food to Fibre Release by Royal Canin which again made no difference. We also repeated the CBC and ordered a serum test. CBC had some abnormalities that vet didnt seem concerned about but serum showed low Potassium and high Chloride.
Then on the 13th of October, my cat had his first "episode" - after my cat pooped, his hair started to stand up so much that he was literally double his size, he would walk very slowly, his head was hanging low and his bum was very raised. Then after a few hours, he would stay in a semi comatose state where he would not respond to anything. Wouldnt eat, pee, poop. His third eyelid also started showing. Basically he was completely lifeless (video here https://www.dropbox.com/sh/5j761hprp...HZFOID28a?dl=0). We rushed him to the vet who performed a series of tests including Xrays, a proBNP test for the heart, and Ultrasound of the abdomen. Everything was normal. The vet concluded thiamine deficiency in my cat since we have been feeding him tuna cat food all his life and was prescribed B1+B6+B12 shots daily plus multivitamins. These however made little difference and two days later when it was poop day, my cat again went into a semi comatose state. We took him again to vet and got another set of tests done: CBC, Haematology, T4 and Cholesterol test, Felv/FIV test, corona virus test, and toxoplasmosis test. Everything came back normal except low LYM and low NA which wouldnt cause these symptoms. The values that were abnormal in the CBC we initally did were now normal.
Now we are at a loss about what we should do. With every vet visit we are even more confused. Furthermore, my cat keeps losing weight. He has gone from 5.6 kgs to 4.9 kgs in a month. A year ago he was 7.5 kgs which is his normal weight.
My cat stays okay until it is poop day when he goes into a semi comatose state which lasts 24 hours. In his most recent episodes, my cats back legs became very weak and he couldnt move them properly. Note that all the symptoms disappear when his episode ends.
Our vet has suggested using various injections and seeing which one works on him so we get an idea about the diagnosis. So far we have tried Dexason (steroid for auto immune diseases) which had no effect on him and Butarphanol (pain killer) which we gave him today. Its been 3 hours and he is the same so I guess this will not have an effect on him either.
We are really confused and are thinking of getting a colonoscopy done since there is a relationship between my cats episode and pooping. If you have ever experienced anything like this please let me know!

Below are tests that we have performed with their conclusions:
19 September
#1 CBC - High TP (8.10), Low ALB (2.12), High Urea (57)
#2 Haematology - High MCV (61), High MCH(19.4), Low RDW (15.1)
#3 Urinanalysis - nothing abnormal
#4 Xrays of the intestines

9 October
#5 CBC -Low ALB (1.97), High Cholesterol(289)
#6 Serum Test - Low Potassium(3.7), High Chloride (129)
#7 Xrays of legs and abdomen - showed beginning of arthritis

14 October
#8 Ultrasound of the Abdomen - normal
#9 Xray of the heart - normal
#10 ProBNP test - negative

21 October
#11 Felv/FIV test - negative
#12 Coronavirus test - negative
#13 Toxoplasmosis test - negative
#14 CBC - Low Sodium (137)
#15 Haematology - Low LYM (0.9, Low PLT (263)
#16 T4 and Cholesterol - High cholesterol (236)

23 October
#17 Urinanalysis - normal
#18 Repeat Ultrasound of Abdomen - normal