You can get low carb versions of spaghetti in the "German" supermatkets, i believe. I use soy based pasta, but there is a black bean variant too. Acceptable substitute i find, but keeps sugar spikes down.
I am not an insulin user, but if the spag bol you refer to is full of meaty protein, then I have seen it suggested that allowing the protein content to be considered to be around half the carbs added on top of the actual carbs when you do a bolus can help.
The other thing I gather is a problem that may be at play here is if the meal has a high fat content then this delays the spikes that would 'normally' occur, and this is why some use a split bolus as you explained, The timing of straight after the meal may be too soon, so as suggested some experimentation to see when the spikes actually happen. You should see the glucose spike from carbs in the first 2 hours, followed up to 2 hours later by the protein bump, but high fat can make that even later, I see this effect as a T2D without being insulin user, so I know it can happen to anyone. You could possibly juggle the fat content but that will change the taste and satiety of the meal so is perhaps a too simplified solution.
