So cheking some damage table son the wiki, i came to the conclusion that it's better to leave all your swords at 4* (the damage differences are of no more than 20 points) and upgrade them later, but first get a krogmo trinket. With the bonus trinket, you'd get even more damage on all weapons at once instead of needing to upgrade them one by one. This may make an impact on some upgrade plans.
Is this true? (to non Skolver + BTS users of course)
I would say that yes, this logic is very sound for a budget-conscientious player. This is especially true since as far as damage output, the differences between 4* and 5* weapons are really minor/non-existent until the 6th stratum. Also, unless you are seeking specific reasons to get the 5* versions of weapons (such as difference in Charge Attack, like with Divine Avenger), using even just one 4* damage trinket will be better in the long haul if you use many swords, and especially moreso if you can afford two 4*, or even go into the 5* versions.
The only catch is if you wanted the trinket slot for something of very similar value, like say a Speed Trinket. Though out of the two, I advocate Sword Damage Bonus over ASI anyway.
Would this actually be more cost efficient in the long run? Eventually Trinket Slot Upgrades costs will exceed the costs of upgrading individual equipments (provided you don't continually get more).
That's true, which is why the trinket is good if you use at least 2 different swords regularly, and actually very useful if you use 3+ different swords.
@Windsickle A full set of trinket and weapon slot upgrades are only about 600ce to buy from players who use mist alts, or 400 if you've got a mist alt of your own.
And in the long run, you'll still have the trinkets if/when you upgrade your swords to 5*.
I don't exactly know, but from what I've seen, each level of damage bonus (Low, Medium, High, etc.) adds 5%-6% to your damage. Using this fact and some of the damage tables on the wiki, maybe you can figure it out.
Another thing to consider is that weapons are weakened, when you use them in shallower strata than they're designed for (e.g. 5-star in Stratum 4). If your goal is to do jelly runs, for example, then 5-star weapons are little better than 4-star weapons anyway. Another thing to consider is that some items become significantly different when they reach 5-star; for example, the DA charge attack gets three projectiles.