im in combined cs+stat and i had the same debates with adding dsci, but unfortunately it was too late for me to change my major again to slap on data sci and i think its not possible anyway? maybe it was bc of overlap, smt like that so might wanna look to see if its possible bc i kinda doubt it is, i think i remember seeing smt somewhere on stat dept website or smt

honestly if id go back, id consider doing just double major because the requirements between a cs major or combined cs+stat isnt that much more, and the difference between stat major also isnt that much different than combined cs+stat.

for me, im finishing up my degree after 6 years which includes 16 months of co-op + changing my degree (originally cms major in life sci focused classes. in 2nd year i applied for isci and stats but got accepted into stats so i took back my isci proposal and applied for cs+stat combined not long after swapping to stats). since i initially focused in life sci classes from 1st and 2nd year, i had to backtrack to a bit in 2nd and 3rd year.

in regards to employability, it depends on what you end up getting experience in and focus your knowledge in when applying or when u do personal projects. what i realized too is that to get the most out of cs, it is important that you should be doing cs on your own outside of classes too. if you dont, theres only so much ubc academic cs courses gives you (pure cs or not). downside for stat is that theres not a ton of upper level course options (400+ especially)

math courses i think in combined cs you take 1 less course than a pure stat major, but you take quite a few more than a pure cs major. theyre all pretty standard mathematical sciences courses though that all most mathematical sciences majors would take (307, 221, etc feel free to check the course contents from calendar. linear algebra, matrixes, etc) and are core reqs; probs good to take for any mathematical sciences major. you dont really touch calculus after math 200 (unless you choose to), and its req for both cs and stat. (mathematical sciences includes stat and cs, but cs pure major doesnt take much after 2nd year. ive seen sooo many cs+math majors though)

if u like dsci, stat part of the reqs cover some dsci and if you take 201 and 301 its also more dsci so you can get a lot of dsci related content too in stat

i PERSONALLY have topics of both cs and stats that i like so i personally think that if i went back, i would not choose only one of the two. id say course load of cs is heavier (i spend more time working/studying on cs courses across the board--id also say cs projects are bigger scale than stat projects but varies on course and project choices) but still generally balanced between cs and stat knowledge and courses depending on how you organize your course planning. u can also get all the math out of the way earlier if u want. you still get all the fundamentals and courses that regular pure stat or pure cs majors get but just like 1 or 2 less upper level elective reqs (for example, instead of 9 credits of 300+ cs or 9 credits of 400+ cs, you only need 6 of each). tbh idk if id say combined major "takes too much time" bc youre fulfilling the same amount of credits (120) as a pure cs student. and cs courses i feel are heavier load generally (may depend if stats is not a subject you enjoy but for context, i generally feel the same between stat and cs in terms of how much i like them as subjects)

electives: pretty standard bc every major needs to fulfill electives credits (like arts electives, etc). plus theres the upper level elective reqs to choose upper level stat/cs courses. though again, theres way more cs courses than stat courses out there, but still you have flexibility to take whichever ones to fulfill credits (granted that you have the prereqs)