Personally, I like to refrain from using TBT for in game things unless they're unorderable and I missed them or they're very grind and I've gotten most of the set (e.g. maybe I'll buy the last two or three mush DIYs I'm missing). If you want the ACNH economy to feel like the intended ACNH economy, barter! That's how I do most of my trades. That way TBT becomes something that you appreciate for how it lets you achieve things that you could've had but missed or don't have the time for (I'm fairly poor in game, but I haven't spent any TBT to get richer, because I like how you make bells in the game, for another example).
Part of what keeps TBT so valuable relative to in-game things is the fact that it's such a liquid and universal currency. You can spend it on just about anything that you and another user might share in common! Want a shiny hidden ability 5IV pokemon? TBT can get you that! On *top* of that, the forum has its own internal collectibles and other things like the beautiful backdrops that can only be obtained via TBT if not firsthand from events.
When ACNH first came out, very few people had star fragments or wands, so you could sell those for millions of IGB or 100 or so TBT. Now that they're more common amongst people, prices have dropped!
It's not so much that things in game aren't valuable, or that TBT is so incredibly valuable, but that TBT will always be something people want, whereas most video game things depreciate very quickly post-release as people become satisfied with what they have, or don't *need* to be the first one to show something off.
Really, I love what TBT lets you achieve in certain games when time might be a constraint! and I also limit myself with how I use it so games like ACNH can maintain an authenticity in certain aspects. The key thing is, TBT exists, it's *quite* powerful
but what you let it do for you is up to you!
edit: just to edit what Jacob said, while TBT used to be exploitable in the past (I think people spam-created accounts to get 200 welcome bells or something like that), it's a very secure currency and has been for a decent number of years, so will retain its value against the depreciating video game things