Act 3 size is like the total of act 1 + act 2. It can definitely exhaust you on the first run. However if you really care about your companions, then you definitely should finish it, or up to the final fight. Because besides Shadowheart, all other companion quest conclude in act 3. IMHO, Asterion and Karlach quest closing are really epic! Wyll's only a little better than Gale and Lae'zel.
On 2nd and later play through, it get easier as you know what can be skipped, what and where to get gear you want. And properly know quest sequence to get the outcome for your companions. Completing the main quest line in act 3 is more than enough to level 12, so experience is not a concern at all.
For the final fight, if you are not into D&D fighting, just barrel the last two fight. We have access to cloud giant strength potion starting act 1, carrying 2 pack is not an issue.
