You can save clips with particular transitions in compound clips that can be opened and copied/pasted later. Not a very appealing solution.
Titles cannot be "adjusted"/customized and saved as presets. They are what they are, and edited versions can be saved like transitions in compound clips...
Effects can be saved as presets and you can copy and paste them across the entire storyline if you like (in "one throw").
If you'd like, you can try using Fade To or From Color
https://fcpxtemplates.com/downloads/
(Please let me know when you have downloaded it or even if you don't want it).
The download includes "Clone Layer" (basically, an adjustment layer that does nothing... but read on).
There are two Styles: To Color (which transitions To the chosen color which will persist according to the timing settings) and From Color (ends of clip start with chosen color which fade away to reveal normal clip.)
Using Style > To Color with Clone Layer allows you to fade to a color then out of the color over the storyline using a title. This makes it easy to *position* the "transition" effect over areas where clips join. Using Clone Layer + Fade To or From Color, it is simple to option-click-drag duplicates as needed to be placed where needed.
When used with regular clips, use From Color. Solid colors will exist at the beginning (optional) and end (also optional) of the clip. Two clips together will behave exactly like a transition.
Timing is in Percent of Length of Clip (%). You can set Start and End time as well as custom lengths of fades. [*If In End % is less than In Start %, the effect will be that the color just toggles (depending on the Style setting) — the same for Out End < Out Start.
Color fades can be eliminated on one end or the other by setting the In End % to 0% or Out Start % 100%.
In and Out Easing is provided: 0% is Linear; 100% is supposed to be Ease Both. Easing will be more noticeable with longer fades.
You can design the color you prefer, in/out offsets and times and Save as Effects Preset. As an effect, you can simply Copy any clip you have added it to, then Command-Shift-V (Paste Attributes) to apply your clip settings to any other clip(s).
Hope you find it to your liking.
