The problem with recent apps and especially with LN is that require some more hardware and software. android 5 is really old, reduced in functionalities and power. The only think I could think will be the old Schildbach, one of the first mobile bitcoin wallets. But that one doesn't have LN, only onchain and I think is quite obsolete, but still works. https://wallet.schildbach.de/ so yes, you could try this one. If you want only for "savings", then just use Sentinel. is a watch-only app. Is simple and could workd. But you will have to create the wallet and extract the xpub on another device/way, offline.
You could try Machankura, you know what they are using on those old phones.
Green will not work either, is too much required as OS level. Custodials? Many: CoinOS, Walletano, LifePay, LNbits all on PWA but are custodials, you said you don't want custodial....
Schildbach look's a great options and still maintained (unbelivable!)
Custodial option better than nothing when accompanied by a non-custodial... we'll try and post updates here.
Thanks again
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What you can do is to use CoinOS or Walletano for receiving LN. And then swap out (they have integrated swaps) into onchain (Schildbach), when the amount is well enough. See if you can generate segwit address in that Schildbach.
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