My design process has evolved over time but I feel I have hit a particularly solid groove that might help inspire your process. This all starts with the design which I tend to just make notes and layout the general approach in notion. This is where I think through each mechanic and try to spot holes in my mechanics individually or how they operate together.
Next I make updates to my cards which I have saved in google sheets. This makes it easy to add new properties to my cards and to make individual card changes without too much effort. Once the cards are in line with my mechanic changes I move on to nanDeck. NanDeck is a free card design software that is pretty straight forward and allows you to setup styles for each deck and pulls in your cards from your google sheet.
This is a huge time saver since I can just design a card for each deck type and it will bring in every individual card in each deck. Once I get my cards designed to work with my new mechanics and make any general design improvements that are required I then use table top simulator to turn the cards into the appropriate format for being imported. NanDeck has export features that make it really easy to get them ready for importing into table top simulator.
Once I bring my cards into table top simulator I am ready to playtest! This process does take a little while but feels really solid for being able to make sweeping changes to my game and get them tested relatively quickly. I currently have been mainly solo playtesting which works but is not going to be great long term. Next step to improve this process will be establishing a more consistent playtest with other people to ensure that I am getting a wide array of opinions instead of just my own. Thanks for reading and happy designing!