Onshape was free in the past I don't know if it's always the case...

You have Fusion 360 who has a free version...https://www.autodesk.com/campaigns/fusion-360-for-hobbyists

else you have also Design SparkMechanical but i's a little tricky! ;) https://moi3d.com/forum/index.php?webtag=MOI&msg=6186.1

123D Design but i believe that it's not more downloadable

All have robust fillets

A little trick about fillet in MOI when that is blocking with a perimeter selected with very little sharp form or angular corners or maximum admissible

Make the fillet admissible then Trim then remake Fillets
and(without to exit the function) a variable Fillet with 0 at the angles if wished! ;)



but with Trim and variable fillets...you can overpass that! :)