Whew, that's a lot of information, and I appreciate you taking the time to "write" all that. I can try and "answer" some of it.
First, the tasks I use my Macs for are much "easier" and "less demanding" than yours. I use Outlook for my EMail, and I am on the internet a fair amount also. My use of Word and Excel are for occasional things, but certainly not every day. I use VLC a "good amount" to view videos, both on line and on my external drives (Movies and TV series, and they are on SSDs). I also use Quicken 2017 (still have Quicken 2007) for keeping track of my checking account transactions.
Probably the most "intense" thing I do (in terms of time and resources) is that every Saturday (sometimes on Fridays if we are going somewhere on a Saturday), I go through my disk cleanup/maintenance/repairs, and backups, processing for both of my Macs. Even though I do a good amount of disk cleanup on my own between Saturdays, I perform the following tasks on Saturdays, for both of my Macs:
1. Run Onyx to primarily do disk cleanup. The tasks I run are the following:
- under the "Maintenance" tab, I run Permissions (it's actually Repair Permissions, and I have to run it at least twice), Scripts, and Rebuilding
- under the "Cleaning" tab, I run Misc., Logs, Fonts, Internet, User, and System (except for Logs, just about the rest of them primarily clean up caches)
2. Run Tech Tool Pro for more "intense" disk cleanup, along with disk maintenance, and repairs (although I rarely, if ever, need to do any actual software repairs).
3. Run SuperDuper! to create two
bootable backups for each of my Macs to the two external SSDs.
Secondly, I actually do not know how to tell where deleted EMails are. I actually learned about this "permanent deletion" business (on my Macs) first with Outlook 2011, and now Outlook 2016. Maybe someone with more knowledge than I can answer your question. However, if your EMail "client: is gmail on the internet, that's a different situation. I also use gmail for "non-important" EMails, and just about all of them are spam, which I delete immediately. I've often wondered if they do get permanently removed (I really do not think such EMails are stored on my internal drive (SSDs in both cases).
Third, as far as upgrading to Sierra (that would be "jumping" past El Capitan, which is/was actually a good, stable OS) and compatibility of software, this site can be of assistance with that:
https://roaringapps.com/apps
Given that I do not use any Apple software (except, of course, the Finder), I rely on my own initiative to insure that all my third party software that I use is compatible with whatever Mac OS I am planning to upgrade to (along, of course, with keeping such software up to date). As it is, regarding High Sierra, the following critical third party software that I have is either already compatible, or already have/had upgrades available:
1Password
Office 2016
Onyx
Quicken 2017
SuperDuper!
AppCleaner (nifty free application that helps with finding all associated "stuff" when deleting an application)
VLC
HandBrake
Transmission
GrandPerspective
The critical one I am waiting on is Tech Tool Pro. In my "visits" to Micromat's site (they develop Tech Tool Pro), I have seen all the issues they are having with High Sierra. Shirt Pocket Software (they make SuperDuper!) also had a number of issues with High Sierra before they came out with V3.0 of SuperDuper! earlier this week.
Third, given that you use a good amount of software that need some space, you might want to consider purchasing a new Mac with at least a 500 gig SSD. Yes, it could be expensive, but it seems like a Mac with a 252 gig SSD still might not be enough for your processing needs.
However, you can take another route. It is not too "difficult" to take the SSD out of your Mac, and install a new one. This site shows how to do it for the early 2015 Mac Book Air models:
13 Inch:
https://www.ifixit.com/Guide/MacBook+Air+13-Inch+Early+2015+SSD+Replacement/38535
11 inch:
https://www.ifixit.com/Guide/MacBook+Air+11-Inch+Early+2015+SSD+Replacement/38511
(It looks like the steps are similar). And within the link for the 13" model, there is this link, which shows where you can purchase replacement SSDs:
https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015
OWC is an excellent company, with a stellar reputation.
Finally, it seems like you have a lot of external storage available. If you want to continue using it (managing it the best way you can; I know that is difficult, especially with all the things you do), maybe a 240 gig SSD from OWC might be enough. But if it were me, even with all that external space, I would go for the 480 gig SSD.
Whew! OK, I think that's enough for now. This discourse is certainly helpful and educational. Please continue to ask questions and/or seek advice. Besides myself, some other knowledgeable folks (and I know there are at least 2 "super genius" folks here) might "chime in".