Thanks for the tag, Terran!
Kierun, let me make sure I understand the vocabulary:
mil TSP - legacy High Three pension, no matching
civ TSP - BRS, FERS, matching
I'm going to assume that you have no match in the military TSP but you get the federal civil-service match in your civilian TSP, right? I'll base the rest of my response on this assumption, so please correct me if I'm mistaken.
Does it make sense to continue to contribute to both even though there's no matching on the mil? Originally, my thought was to contribute to both because the mil balance is quite low, but does that really matter? It's been making the mathing to stay within the IRS contribution limit a bit annoying, especially recently with unexpected covid activation orders and such.
As you've noted, your elective deferral limit on 401(k) contributions ($19,500 in 2020) is tracked by your Social Security Number on your W-2s (for the IRS). The EDL does not include employer matching contributions (from DoD or the federal employer) so you can contribute all $19,500 on your own.
https://www.tsp.gov/making-contributions/contribution-limits/You can split up your $19,500 on contributions between the two accounts any way you want, as long as the total does not exceed $19,500. The TSP system doesn't have the feature to track your separate contributions by SSN between two accounts-- only the IRS does that from the W-2s-- so it's up to you to avoid the IRS automated query letter.
As you've also pointed out, it doesn't make any difference to contribute to both. You'd want to maximize your match in your civil-service account, but you don't have any compelling reason to contribute to your military TSP account as long as you can reach the EDL (and get the full FERS matching contributions) from your civil-service pay.
When you're contributing to your civilian TSP, you'd still want to contribute in all 12 months of the year in order to receive the match. If you front-load your civilian TSP and reach the EDL before December then the TSP will cut off further contributions and you'll lose some matching contributions.
If you're receiving combat zone tax-exempt pay then you can contribute more to your military TSP (up to the annual addition limit, $57K in 2020). That's a whole different messy discussion.
Does it make sense to continue to contribute to both even though there's no matching on the
a) Drop contributions to mil and just max out the civ for simplicity and all that?
b) Continue to math out contributions to both to stay within IRS limit?
c) better ideas?
I'd do a). The only time it'd make sense to do b) would be if you mobilized on active duty and (for some reason) your civil-service pay completely stopped (no more matching contributions, either). If you still wanted to reach the EDL then you'd have to turn on your military TSP contributions (and do your math again).