It's amazing how many players believe that going from Thomastik flats to heavy gauge drop tuning rounds won't have much effect on the neck.
I find even switching brands, still the same gauge, will require a truss rod tweak.
Difficult situation now... I like the Spark and Beta MKII in different ways. The Spark is more neutral and organic sounding but the Beta gain and break-up sounds more valve like... sigh! Beta reminded me a lot of the Bearfoot Blueberry.
Apologies, didn't even mention the bass guitars in the original post.
2016 Fender American Standard Jazz Bass installed with DR Pure Blues.
2021 Squier 40th Anniversary Precision Bass installed with Dunlop Flatwounds.
I have two basses which cover all eventualities. A P with flats and a J with rounds. The J was my only bass for the last couple of years. The P was going to be a reserve but is now on equal footing. Who else is done?
I highly recommend the Neotech Mega.
https://www.gear4music.com/Guitar-and-Bass/Neotech-Mega-Bass-Guitar-Strap-Regular-Black/23VG?origin=product-ads&gad_source=1&gclid=EAIaIQobChMI3ZPs6YCPgwMVDTgGAB26uASVEAQYASABEgK_AvD_BwE
I've used one for over five years with no issues. Better made than the Comfort Strapp and does not slide like the Mono GS1.