Based on your post, you seem satisfied with the 401. You don’t seem like an OCDed audiophile who is looking to compare each attribute of different tables and find something better. The question you are asking is going to solicit a lot of different opinions, and you can spend a fortune validating those opinions, or just pick your random choice of internet stranger, including one who agrees wholeheartedly with you that Garrards are the best.
It also depends on who has restored your TD 124 vs Garrard, as that can influence preference.
Those who restore both Garrards and commonwealth will tell you that the CW is better than Garrards on all fronts, just more of the same.
With belts, there are many belts, you can spend all the way upto 450k on belts (Some poor or unnecessarily premium-ed ones across that range, some with certain attributes spotlighted that you might like). Lots of variations and combos possible there.
If budget is a concern, I wouldn’t change, I would keep the Garrard and save up to add the DaVa field coil cart on it. It would give you one of the highest resolutions possible through an analog front end without losing out on what you like about idlers, especially given that the Dava designer uses both Garrard and Commonwealth.
Personally if I had a lot of money I would buy two tables, the Vyger (belt drive) with the Red Sparrow cart and a restored Commonwealth with the DaVa field coil cart.
If budget was a concern I would stick to Lenco/TD 124/Garrard with a SPU, and with more money add the DaVa which fits on same headshell as the SPU. If you were mostly into rock and amplified, and/or are not into spending on high quality LPs, you don’t need any upgrade beyond this. Even for classical, only the most OCDed need another TT.
Audio Silente himself make the graphite based Blackstone idler which sounds quite better and much higher resolution than his TD 124. I cannot tell you how it compares to belts