Do have a temp controller and chamber for fermentation? That is WAY more beneficial of an item to have than an in-kettle thermometer/brewmometer. I would put the money toward that. You can always add the no-weld brewmometer later.
I use a thermapen for my BIAB batches, which works great. I will say that having a great thermometer is money really well spent. With thermometers, I am a firm believer that you can spend $100 over a number of years/months, brews, frustrations, and poorly-cooked meats, and wind up with a drawer full of junk, or spend the $100 or so on one of Thermoworks’ products and avoid the headache.
EDIT: I actually have an in-kettle weldless brewmometer in my kettle and still rely on the thermapen. The only time I really use the in-kettle is when chilling the wort.