I don’t own, nor have every played a Mainland, but I can tell you about some of my current favorites...
On my laminate mahogany Kala KA-T I really liked the Aquila REDS, but right now I have a Living Water re-entrant set installed, and there's something almost magical about the way that this instrument responds to these strings. I also had the Oasis 'bright' re-entrant set on them, and while very close, they did not have as much sparkle in the bell-like tone of this same Kala.
On my tenor Fluke with a solid Koa top, I currently have Martin M620 strings, re-entrant, but tuned DOWN to E-A-C#-F# and at this tuning, the instrument has nearly infinite sustain and both a sweet and bell-like tone.
On my tenor Hibiscus Flea, with the solid Birch top I currently have Worth Browns Tenor (BT) set installed, and on this instrument, these are the best sounding strings to my ear with both a dulcet, yet warm tone, with a bit of sparkle.
One my tenor Walnut Flea, with the birch rosette, I currently have the standard Aquila Nylgut re-entrant set, and on this instrument it has that simple midrange punch, that sounds/feels like a sucker punch to the gut, much in the same kind of sound of a banjolele, and this helps for cutting thru and getting heard in a group setting.
For a low G, you will probably not get the best sound or tension with an unwound string. I have literally tried them all and find them too floppy for my taste. I used to use the Fremont Soloist wound low-G which sold as a single string, until fellow UU brother OldePhart (John) intriduced me to the Thomastik-Infeld CHROME FLATWOUND classical strings, which are the smoothest surface of any of the dozen similar strings I have tried, for a low-G, I have used as my go-to, either the CF27 or CF30 string, which are the 'D' and 'G' strings for classical guitar, and depending upon the tension you want, and the acoustic response of the specific instrument, either will work very well, and typically be thinner than all unwound low-G strings you will find which are typically 0.0358" no matter what brand. the Thomastik strings are 0.027" and 0.030" respectively and as such you will probably NOT have to enlarge the nut slot for your G string to use them, as I have not, on any instrument so far where I have used them as a Low-G.
Also, for a while I was experimenting with a wound-C string, and the CF27 worked well there too, albeit at a higher tension than the remaining 3 (unwound) strings.
All of the above comes from testing more than 40 different string sets, from over 12 different string makers.
If you use my referral link in my signature, I get points to buy more strings for myself and you get a discount off your first order.
Strings and Beyond does not carry the Thomastik-Infeld single strings, nor any Fremont, so you'll have to go to Strings By Mail or Just Strings for those (and I'm sure other vendors might carry them too, like HMS and Elderly).
Can of worms indeed, but BEWARE FOR you know you have a serious problem when you need to buy more ukes, even duplicates of an existing uke because you like more than one string set on them, and for DIFFERENT reasons and DIFFERENT sound (OR DIFFERENT TUNINGS), when the String Changing Obsession (SCO) is actually a driving force behind UAS...has not happened to me yet, but I can see things progressing to that point for someone who becomes lost to the obsession.
[yes I am also mindful that many ukes of the same exact brand and model CAN and usually will sound very different from each other, since 'all things being equal' is rarely achievable in this case since wood is imperfect and varied by nature due to being organic, at the very least.]
Hope this info helps.