I think tg are not really an issue for the game and ladder longevity ... i think a few days ago on discord there was a discussion related to ssf ladder even (which i would welcome and probably play, but i am not sure is a good idea given the thin player base).
related to the tg credit, i don't think it is a good idea. as you said, it can be exploited by using multiple accounts, and it's also quite complicated and convoluted. instead, i think that blocking tg trades - or trade in general - for the first week (or another time limit) could be more effective. also in this case, there is the issue of people who could just trade the items and send the tg later, after the deadline expires. in short, there is really no way to limit the use of tg in any of the suggested ways, unless there is a control system in place and bans coming and tg removals for players going against the rules. if it is possible to check if there are breaches, then instead of a tg credit you could limit the amount of tg usable in a season to a certain amount (for example, 1k .. random number).
but in my humble opinion, the main issue is not tg, but it is trading charms. trading charms should not be allowed. I mean, charms shall be bind to the character who earned them (not even the account, ie. not possible to use on any other character apart from the one who earned them).
now i see this as the main issue for longevity but also for the competitiveness of the ladder. tg is secondary.