Discretionary ones are probably the most complex to set up along with will trusts. Has a solicitor drafted the trust deeds yet?
How many trustees are there? Are they induviduals or a professional firm (solicitor for example). Each trustee will need to be AML verifified. So this can become quite complex if you have lots of trustees.
If the beneficiary is a person, depending on their age they may also need to be AML verified.
This is where you are going to get the most trouble, lots of online services it’s quite easy for them to say joint holders in the same house have agreed to be AML verified as part of an online application - kinda hard to prove that all the trustees are in the same location to do an online app, so most will opt for post applications.
Once you do have the accounts set up, check if the providers offers ‘joint renunciation’ for trust accounts. If they do, then one person will be able to act on the trust freely without all the other trustees to sign instructions together all the time. This will often or not also give you online access. Without this, most companies will just not provide online access.
Most banks will set up a trust bank account, and yes, you can then link them to an investment platform.
The next hurdle you are going to face is ongoing admin, trusts are usually forgotten, and the biggest headaches I come across is when a trust has changes rules or trustees and when they try to act on the trust they cannot because they have not notified the banks or investment firms of the changes.