Have you tried instead editing a existing pokemon into "Egg" By setting it's species id to the one immediately after the last known species in the game? In gen 6 and 7, this results in an "egg" that you can use in battle. Though in gen 7 the game crashes if you attempt to use it in battle. In Gen 6 (Pokemon X/Y and ORAS) the egg was functional in battle but had base zero stats so was pretty terrible.
You'll have to modify PkHeX to allow creating such a pokemon as that's what I had to do with Gen 6/7. I'm not sure what that would be for these games. May either be the species index after maltan/malmetal (that new nut pokemon.
) or the one after pokemon 151/152 (which ever one Mew happens to be). Looks like you've tried something like this via the evolution tables in the rom instead. This may cause crashes. Once PkHeX is available for this game, try that route instead.
In gens 6/7 all mons you make after the egg one results in mostly bulbasours with a mix of ivysours every now and then. Looks like this game uses marawaks?
Don't use species index 0. In gens 6/7 the games treats that as an empty box/party slot so what you ended up with is a malformed pokemon of some kind so that may be why your game crashed immediately. Try what I did, you may get more stable results. You'd at the very least be able to view it in your party. (would be interesting to see what would happen if you tried to set it as your overworld follower). Though it does seem that "egg" in gens 6/7 used the base stats from species index 0. If I modified those stats for species 0 in the rom, "egg" would take those stats (and the TM/Learn move pool too so I could have it learn moves if I wanted too).
You could instead try and make "Egg" appear via the wild encounter tables. That would be interesting. Wonder if that would work seeing that overworld pokemon are a thing now and wild encounters are shown via that. Assuming it wouldn't crash from that you'd see "Egg" running around prior to encountering it.
Normally I'd advise a masterball to catch it (because it uses stats from species 0 and which has really really bad near impossible catch rate. Might in fact be impossible? The value for catch rate is 0 or 255. Either the lowest possible or the highest. I don't recall.
) "Egg" has no move pool and will just struggle to death if you try to waste normal balls on it. Don't really battle wild mons in these games though so that may not be a problem. Might still need a masterball but not sure. Depends on how they reworked the catch rate stuff in this game. Are masterballs even in this game? lol