Preferred Visa Application Center/Centre and general advice!

After about 3 months into our relationship we moved in together in Taiwan, lived together there for about 3 years, then moved to the UK where we both stayed with her parents for 6 months until I had to leave to wait for the spouse Visa, which was about 2 months ago.

Okay, in that case, you will need 3 years' worth of evidence of living together in Taiwan, by way of mail received by both of you at the same address for all of the 3 years. Things like:
- tenancy agreements
- utility bills
- bank statements
- tax returns

Then for the 6 months you were in the UK, as you were not legally living there (you were just a visitor in the UK) you will need evidence of your 6-month trip to the UK, for example:
- boarding passes for your flights to and from the UK

You will also need:
- evidence of regular communication between you for the 3 months before you moved in together
- evidence of regular communication between you for the last 2 months since you left the UK

The regular communication evidence could be things like:
- screenshots of emails/calls/messages between you (maybe 1 or 2 per week)
- any letters or cards you sent to each other

After about 3 months into our relationship we moved in together in Taiwan, lived together there for about 3 years, then moved to the UK where we both stayed with her parents for 6 months until I had to leave to wait for the spouse Visa, which was about 2 months ago.

Thank you! So you'll need emails, chats, Skype  history or whatever you used to keep in touch for the first three and the last 2 months.
For the period you were living together, you'll need paper proof of joint life as mentioned, covering the full 3 years.

For the UK 6 months, it's a little trickier because weren't living in the UK, you were visiting. Maybe include tickets from day trips or other activities you've done together.

Include boarding passes from trips you've taken together. And don't forget letters and cards, they like to see those!