Quick question:
If you have a talisman which you open when you are young, the amount of Vis put into the Talisman is limited by your two highest arts instead of your Lab total.
Lets say that 20 years later your two highest arts have significantly increased. Can you add more capacity to your Talisman as your abilities improve after the initial opening?
Your vis handling in a single season is still limited by Magic Theory. So you need enough Magic Theory to cover the initial opening of the item, but not to reach the capacity in one go. Successive seasons however can add up to your maximum allowed capacity in the Talisman. The capacity for the total vis invested into your Talisman I believe is intended to scale with your increasing Arts over time.
It's for a lab focused Bonisagus, so magic theory isn't much of a problem.
When you open your Talisman, your upper limit might be your Two highest Arts, but that's not necessarily how much opened slots are in it the Talisman, since that's limited by your MT x2.
As your highest Arts increase, the upper limit on how much Vis your Talisman can contain, also increases, but you need to occasionally spend a season to add Vim Vis to take advantage of that potential increase.
Yes, you can "top up" your talisman capacity as your abilities grow.
It is a nice change from previous edition, where for your talisman as well as your familiar, a mage had to decide how long he was willing to wait to get one since it was "once and done". In 5th ed, both your Talisman and your familiar grow with the mage ability, at the small cost of a season occasionally.
@Ezechiel3571 already answered this, but to make things very clear:
Yes you can. That's one of the main reasons why Talismans can remain relevant as your magus grows in age and power.
But bear in mind that when you first enchant the item, you haven't yet made it your Talisman, so you are limited by size and material. (As soon as you've made it your Talisman, you can increase that number. But you might as well do enchantments with its current capacity first.)