How about adding another downspout ?
Not too difficult.
This is about a 40-ft run of gutter so it probably should have 2 downspouts - if only I could. The existing downspout is at a corner of the garage and outlets to my back lawn. From there, the gutter runs across the back of the garage, across the breezeway, and abuts the house. Everything below is concrete except at the existing downspout. The only way to install another downspout would be to cut the concrete and install a drain line. That ain't happening. The good news is that it drains a relatively small roof area (just back half of garage & breezeway) and hasn't been overwhelmed by even the worst storms in the 32 years I have lived here.
Self leveling compound would only make level, and the weight alone will rip out the gutters. You must lift the gutters to about 1/8" or greater per foot to work properly. Gutter lifting is a common, easy repair.
I don't know yet how much a cement leveling compound weighs when cured - I have inquiries out. However, something like "Fix-a-Floor" looks like it might work pretty well and the liquid weighs only 20% more than water. That certainly would not be enough to over-stress the gutter. In fact, my concern about using this product is that it is water based and once cured (i.e. presumably water evaporates) it might be lighter than water and float. What I need is a very low viscosity product that cures with a specific gravity about 1.2 - 1.5
Writing the above gave me the idea to check specific gravity of cement; it is about 3. That means filling the gutter to level with SOLID cement would add the equivalent of about 3 inches of water (2 x 1.5 inches) above what it would weigh when full of water. However, the cement wouldn't be solid. The viscosity of products I have seen on youtube is so low that I would not be surprised if they were at least 50% water. Saturated during a rain event, would actually only add a weight equivalent to about 1.5 inches of water. I suspect the gutters could handle that, and certainly could if I add a couple of extra hangers - especially as that maximum weight only occurs at one end. Hmm... I think I might have talked myself into something.
BTW, yes positive drainage is better than a flat bottom, but a flat bottom should drain sufficiently that I won't have a mosquito farm.